I gained weight in basic. But then I was air force.
Enlisted in 69 at 5-7 140 lbs when I got out of Jump School I was 160 lbs. When I was discharged in 72 I was back down to around 140-145. FSB Bastogne and the A Shau Valley was not noted for it’s culinary expertise.
I gained weight as well, in Navy boot camp. But I came in a skinny, 5’6” 109 lbs. Hardest part of boot camp to me was carrying that full seabag from the receiving barracks to the regular barracks. Damn, that thing was heavy.