Something that's plump has a nice round shape. You could describe watching your plump little cousin eating one plump grape after another.
Objects and people can both be described as plump, as long as they're full or round or chubby. Though some people might be offended by being called plump, it's meant to describe an appealing roundness. And when you plump something, you pat or shake it so that it becomes fuller: "Let me plump those pillows on the bed while you tuck the sheets in."