They're not simply fat adults like fat adults in our world. Over 700 years in space, through bone loss and not having to do ANYTHING for themselves, such as walk, they've devolved into, basically, big babies. If you look at them, they look like babies. Stubby limbs, stubby fingers and toes, fat, etc. When the Captain stands up near the end of the film and wobbles a little, it's like when a toddler first learns to stand up and then walk.

And no, I did not come up with that. The director did. I'm not that smart.