Not strange at all, as marshmallow stated, the fetish encompasses a great variance of desires.
There was a documentary I watched a few years ago and I cant, for the life of me, remember the title, but it was about discovering the root of fetishes. For some a fetish develops for the forbidden; your case for example: though you don't want to get fat the idea or thought of being encouraged gets you off. You want it but don't want it. It's not strange, it's pretty normal.
My best girl friend is ndecisive like that either: she loves have to stuffing herself, to feel bloated, to gain significant amount of pounds and consider the fancy about being sizably obese alluring... but she simultaneously fear getting fatter and has somehow image-troubled when we talk about stripping off herself from thin-centered beauty standards and everything it include socially speaking... also, peer pressure doesn't help.
Unless you simply like being tempted to gain some weight for bulking up in extra muscle. Half of my family is like this.