I really enjoy BL2, Ive played it on multiple systems now, and from my experiences, to get this achievem....er...trophy, its totally random. Dont even worry about it, a Chubby will eventually appear. BUT, One thing Ive noticed though is, they usually appear when a lot of bad asses are around.
Skags, rakk, spiderants, varkids, stalkers, midgets, and skeletons(DLC). Are all variants of the chubby you may encounter. You will recognize them immediately as they are much CHUBBIER than their brother-en.
Note: Chubby enemies have a higher health and strength, as well as high experience and loot output upon death.