Having cruised on a carrier and known folks who deployed on land overseas, this is not unusual. Everything is contextual, and when you put some folks in the context of deployment, they decide the remedy for boredom is to work out a lot. Or their job requires it (infantry, hauling heavy things, etc). Other people don't have either of those, and just eat, do their jobs, and sit around.
I have noticed overall that the tendency seems to be for most folks to get trimmer, just because in general you're walking/moving around more and there's generally no alcohol available. Going up and down ladderwells to get everywhere burns calories even if you're not a gym rat.
That said, some sailors consistently come back bigger, and some consistently come back smaller, so it's mixed. :)