“Chubby” IS fat -- it’s just a matter of degrees.   It used to mean somebody with a little bit of “baby fat” on them, but the deathfats and career fats highjacked the word a long time ago and it’s long since become a self-complimenting euphimism that means “FAT” -- if not fatter than all flaming hell.   These days the meaning of that word has been destroyed by fat-worshipping linguistic appropriation.