The quality of something being round or curved in shape is its convexity. You could describe the convexity of a round, squat vase.
The noun convexity is somewhat technical for everyday use — you're more likely to see it in a math textbook than to hear a friend use it to talk about a goldfish bowl. Still, it is a perfectly good way to describe anything that curves or bulges outward, whether it's the belly of a Chinese Buddha or a balloon. Convexity comes from the adjective convex, with its Latin root word convexus, "vaulted or arched."