SCOTTSDALE, Ariz. -- Two weeks ago, Giants manager Gabe Kapler and Kervin Castro were watching Carlos Rodón throw a bullpen session at Scottsdale Stadium. As he stood next to a considerably slimmer Castro, Kapler joked that the rookie pitcher’s nickname might need to be transferred to the burlier Rodón.

“I’m like, ‘Hey man, that might be The Tank,’” Kapler said. “‘You might not be The Tank anymore.’”

Castro’s moniker doesn’t fit as well after he dropped 20 pounds over the offseason, but he’s still making a name for himself in Giants camp, where he’s emerged as a spring standout due to his improved physique.

“Castro looks amazing,” Kapler said. “Castro’s done some really good work. He’s improved his body composition. He’s still plenty big and strong but he’s done a nice job of making a transformation and coming into this camp in great physical condition.”

Castro, 23, didn’t give up an earned run over 13 1/3 innings in 10 relief appearances after debuting with the Giants in September, earning a spot on the postseason roster as a result of the impressive stuff and poise he showed on the mound. To build off that success, Castro decided to focus on slimming down this winter to better prepare his body for the rigors of his first full Major League season in 2022.

“I knew that it was going to be a longer season,” Castro said in Spanish. “I wanted to get in better shape to be able to stay strong and healthy for the entire year.”

Castro said his transformation resulted mostly from diet changes, as he tried to eat better and find healthier alternatives to many of his favorite Venezuelan dishes. The hardest part was cutting back on his beloved arepas, though he still allowed himself to eat a lighter version made with oats.

“More than anything, it was improving my nutrition,” Castro said. “I kept working out, but the biggest change I made was with my diet. I started to take better care of myself.”

Castro said he has better flexibility and endurance on the mound after slimming down, which could set him up well for a role as a bulk-innings reliever or possibly even a starting option this year. He relied on his fastball-curveball combination last season but he’s in the process of trying to learn a third pitch and has been toying with a cutter and a changeup this spring.

“If he was to repeat exactly what he did last year, we’d be over the moon excited,” Kapler said. “It may be that we want to stretch him out a little bit to see him take down multiple innings and be an option for us in that regard. We don’t really know what we’re going to need on our Opening Day roster yet, so it’s hard to see this path perfectly for him. But he did come into camp in great shape. He’s a guy that really embodies the push the pace, pound the zone, know your plan thing. There’s some conversation around whether he's going to work on developing a cutter or if he’s going to continue to work on developing his changeup or both. But the fastball-curveball combination we know is going to play well at the Major League level with tons of strikes.”

Castro was a starter in the Minor Leagues, but he shifted to the bullpen to accelerate his path to the Majors last year and said he’d be open to any role moving forward.

“I’m available for whatever the manager needs,” Castro said. “My dream is to keep playing in the big leagues in whatever way possible.”