It is a bit from both categories, probably. My observations are that men who acknowledge their own status as inferior develop a fixation with women who are not conventionally attractive because they possess an underlying presumption that a less than prototypically hot woman will be easier to attain and thus their standards evolve through time to revolve around this idea.
You probably like chubby girls in part because you think they will undervalue themselves sufficiently to settle for you and whatever inadequacies you may have or believe yourself to have. This is not me flaming you, just playing armchair psychologist.
It's kind of a close cousin of the phenomenon where guys will become obsessive over homely looking or frumpy women because they think that the unattractive qualities of these women will make them easier to get, and that they will not have been as tainted by the desire for Chad that is endemic to most hot girls.
The unfortunate truth is that actually all women are like that, even the fat and nerdy and geeky ones with glasses and mousy hair.
A fat girl might settle for you, but deep in her heart she still desires an idealized concept of Chad.
But it's not really my place to judge. Just jerk off to whatever gets you off and stop dealing with it. Acceptance of reality is the ultimate and only truly effective cope.