I'm not too chubby but even then I find myself doing that every now and then. I don't know why it calms me down, like a stress ball. Rubbing, molding, poking etc.
I love jiggling myself its more of a comfort thing for me. But sometimes just for entertainment or boredom or just cos my husband is watching me. But theres something about the way fat moves
Mine is more of a hard basketball rather than a jiggly apron style one. I do want one of those, though :-) My wife has one of her own and I play with it quite regularly!