In Miami, Marlins Park is "one of the worst hitters parks in the league," Christian Yelich said to local media last spring. In Kansas City, Lorenzo Cain's now-former home in Kauffman Stadium had the second-longest average home run distance (409 feet) in the Majors behind Coors Field, and that's not because it creates long blasts -- it's because the massive size of the outfield demands them.
Neither one has to worry about such things any longer. After Thursday's pair of big moves, Yelich and Cain get to trade in their pitcher-friendly home fields for the hitter-friendly confines of Miller Park.
It will help them both, probably. But how much? Are we talking 30-homer seasons? Let's see if we can find out.
Yelich 
To look at Yelich's surface-level stats, you'd think that he's going to immediately get a great boost from the move to Wisconsin.
In 2017, Yelich slugged just .391 at home, but .484 on the road. It's the difference between Jason Heyward and Corey Seager. For Yelich's career, it's about the same: He has slugged .396 at home, and .465 on the road. The outfielder no longer has to spend time in Marlins Park, and that's a good thing.
It's a little more complicated than just, "Marlins Park is tough for lefty hitters," and we'll get to why in a minute, but let's start with the fact that Marlins Park is tough for lefty hitters, far more so than Miller Park.

There's a million different ways to show that. You could simply point out that lefties slugged .438 (10th-best) in Milwaukee against only .391 (27th-best) in Miami, and that's something, but it's also affected somewhat by the players involved; surely Dee Gordon is going to slug less than Eric Thames and Travis Shaw no matter where they play.
You could look at the difference in expected outcomes (Expected wOBA, a Statcast™ metric that shows the likely production based on the quality of contact of exit velocity and launch angle, regardless of park or defense) against actual outcomes (actual, real-world wOBA). Looking at the difference between the point of contact and the actual outcome should tell us something about how the ballpark affected batted balls.
We'll stick to lefty hitters on fly balls and line drives, and we find that since 2015 at Miller Park, lefty batters were expected to produce a .540 wOBA, and instead received a .598 wOBA. That gap of +.058 points of "extra" wOBA is the fourth most in baseball, behind Colorado, Houston and Cincinnati. In Miami, lefty batters were only expected to put up a .519 wOBA in the first place -- fourth-lowest, which may say something about the hitting conditions. Their actual outcome was essentially unchanged.
It's the same for pitchers. Against lefty batters, pitchers in Milwaukee allowed damage +.007 points worse at home than on the road, the fifth-highest gap. Meanwhile, Marlins pitchers allowed a .326 mark at home against a .343 mark on the road, meaning they were receiving some home-field advantage.

Let's go deeper. The best way to do this isn't to just look at venues, it's to compare teams at home to themselves on the road, which helps to neutralize to some extent the differences in talent you'd see in a Gordon-vs-Thames case. We did this going back to 2015 (excluding Atlanta, since the Braves changed stadiums in that time), looking for now just at lefty hitters.
Of the 29 non-Atlanta teams, lefty Brewers batters had the seventh-largest boost at home, producing a .339 wOBA at home against .317 on the road, a gain of +.022. (Unsurprisingly, Colorado and Arizona were two of the six teams ahead of them.) Lefty Miami batters, meanwhile, were fourth from the bottom, hitting .322 at home and .327 away, which is to say, a difference of -.005. One park seems to help, while the other is neutral at best, damaging at worst.
So in the sense that Miller Park is more friendly to lefty hitters than Marlins Park is, Yelich ought to get a boost. But there's more to it than that, because Yelich has to actually take advantage of it. In a world where we keep hearing stories about hitters trying to elevate, Yelich has done the opposite, mostly. Among 261 hitters with 100 balls in play, he had the 16th-highest grounder rate, 55.6 percent. His launch angle was just 4.7 degrees, where the Major League average was 11.1 degrees.
That's partially why, when we look at his 2017 home batted balls of at least 300 feet (i.e., potential homers) to see if he'd have added more in Milwaukee, the difference is negligible, because there just weren't many. Yelich had 36 such batted balls in '17, similar to J.T. Realmuto or Derek Dietrich. Stars like Manny Machado and Nolan Arenado had 82.

This image transfers Miami batted ball paths without adjusting for Milwaukee's environment, so it's not to say that he would have performed exactly the same so much as it is to point out that grounders don't leave the yard. Yes, the park may change things, but the approach has to change too. This is why Yelich at home last year had an Expected wOBA of just .332, against .372 on the road. No, the park didn't help once the ball got in the air. But his contact in Miami just wasn't impressive in the first place, compared to his road skills.
"My approach up there is to try to put a good swing on a pitch, and see what happens," Yelich said last spring when we asked hitters about the home run surge. "I know a lot of guys are talking about swing up on the ball, and get it in the air, all that stuff. I just don't think about that stuff when I'm up there. I'm just trying to put together a good at-bat. I guess, if my ball goes at a good launch angle, it's going to get a good result."
It's possible that in Miami, he didn't see the benefit of trying to elevate the ball. In Milwaukee, the incentives have changed. The approach can change, too.
Cain
It's a little different for Cain, because he's not generally considered to be a power hitter. He did have an odd 2017 in terms of home vs. road, hitting 12 of his 15 homers away from Kansas City, but since joining the Royals in '11, he's actually hit slightly better in Kauffman Stadium.
Home: .302/.346/.438 (.340 wOBA)
Road: .277/.338/.406 (.324 wOBA)
Though Milwaukee's left-center is considerably shorter than Kansas City's, it looks like only one ball last year may have cleared the wall if it had been in Milwaukee.

The one potential home run you see there is a ball that went a projected 390 feet for a mere double against Alex Wilson and the Tigers on May 30.

There probably is a boost here, too, but less of one than you'd think, and it's more about escaping Kansas City than it is about arriving in Milwaukee.
Looking at righty Brewers batters since 2015, they had a .311 wOBA at home, and .310 on the road, basically meaning there was no change at all. Brewers pitchers against righty pitchers were actually slightly more effective at home (allowing a .316 at home, and .326 away), but the difference there is minimal. Flipping to some other sources for a smell test, Baseball Prospectus has the 2017 home run factor for righties at Miller Park as 102, where 100 is average, making it 16th -- or middle of the pack. Statcorner calls it 107 for righties (and a massive 127 for lefties).
Kansas City, meanwhile, is a lot harsher, coming in at next to last at Baseball Prospectus for righty home run power and last at Statcorner. There's a reason why it took nearly three decades for anyone to break the franchise record of a mere 36 home runs, which was Steve Balboni's until Mike Moustakas broke it last year.
So this benefits Cain, too, though somewhat less than Yelich. You might actually wonder if the shallower fences (and shallower fielders) might actually cost him some of his famous flare hits just over the infielders to the area known as the "LoCain Triangle," because he does that a lot. Among right-handed hitters, only two (Brandon Phillips and DJ LeMahieu) had more batted balls classified as a flare or burner last year.
If you're looking for grand takeaways, it's this: Both Yelich and Cain should benefit some. While Cain's additional upside is somewhat limited, given his tendencies, age and batting hand, Yelich has the opportunity to do something special. He just has to get the ball off the ground a little more; the park might do the rest.