Mattingly said Puig showed up at camp weighing 251 pounds, 26 more than he weighed at the end of the regular season.
Puig, who is a muscular 6-foot-3, was 235 pounds at the beginning of last spring. The Dodgers are hoping Puig can lose some weight before the start of the season on March 22 in Australia.
"We don't feel it's going to be a problem, but we're paying attention to it, put it that way," Mattingly told ESPNLosAngeles.com.
Puig, 23, has been bothered by shoulder soreness and missed the last two days of baseball activities after fouling a ball off his right leg in batting practice. The National League Rookie of the Year runner-up batted .319 with 19 home runs and 42 RBIs after his early June call-up.
For Puig, February began with a public apology for his most recent reckless driving arrest. The charge was dropped by the Florida state attorney's office last week, but Puig still was ticketed for speeding.