bellied
Curving or bulging outward in the middle like a belly.
Bellied describes something that bulges or curves outward, especially in the middle, like a belly does. When a sail fills with wind, it becomes bellied, pushing outward in a smooth curve. An old wooden barrel might have bellied sides that bow gently outward before tapering at the top and bottom.
You might read about pot-bellied stoves in pioneer stories: these wood-burning stoves had a characteristic bulge in their middle section. Ships throughout history have used bellied sails to catch the wind effectively.
The word can also describe someone with a prominent belly, like a big-bellied king in a fairy tale. When used this way, it's usually part of a compound word (like pot-bellied or round-bellied) rather than standing alone.
In nature, you'll find the word describing shapes: a bellied cloud swollen with rain, or the bellied body of a well-fed snake after a meal. The key idea is always that outward curve, that gentle bulging shape that makes something look full and rounded.