The 2015 Willie Mac Award winner played the season between 170 and 175 pounds after getting up to 180 pounds prior to Spring Training. He said he hopes to increase his weight to 185 pounds before 2016 rolls around.
"I think that's a realistic goal," Duffy said. "My first thing, though, is not losing my length and my speed. ... I do want to gain strength, I've just got to be smart about it."
The rookie third baseman believes the added weight will help him become even more agile and that additional power is just a bonus.
"A side effect of that would be more power, but it's not necessarily to hit more home runs," Duffy said of his offseason goal. "It's just to have more explosiveness in general, whether it's throwing or hitting. Base hits that guys are diving and catching get through or balls that are cut off in the gap might go for a double."
Duffy's schedule during the winter months is simple and consistent. He begins training at 8 a.m. and continues his workout regimen until 2:30. He saves the remainder of the day for relaxation, but said he often struggles to stay occupied.
Duffy, who turns 25 in January, wants to find a balance in his workouts over the break. He voiced his concerns about putting on too much weight too quickly, which can lead to injury.
"I don't want to get too bulked up to the point where my muscles shorten up and I lose the elasticity," Duffy said. "Baseball's not a brute-strength sport. It's about quickness and reactiveness. I think that's the key is just keeping that while gaining weight. I think when you gain weight rapidly, that's when you start getting tight and bound up."
Duffy never homered in college, but had 12 home runs entering Saturday to go along with a .297 batting average. His 90 hits since the All-Star break led the National League.
Duffy, who has become one of the Giants' best hitters, is confident the power will continue to increase with time.
"Every time I try to hit a homer, I get out," Duffy said. "I'm just trying to hit the ball hard and keep the ball low, especially [playing at AT&T Park]. ... So that's kind of my approach: just get on base for the guys behind me because they're the ones who can hit 20 homers. I'm not saying I can't one day, but I'd rather be a good hitter first and then learn how to hit home runs later on."
Oliver Macklin is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.