That is interesting.
I would guess you had a positive experience that someone centred around someone's bicep or biceps once and your brain just filed it away as an important association.
It would be really interesting to see how this happened.
I don't really think fetishes are developed through negative associations wholely, like in the sense that you like biceps because you lack them.
It is possible, or even likely, that they would and could develop through some sort of negative experience that you coped with.
Example: When i was 6, a kid beat me up pretty badly by tackling me and punching me over and over again in the face. All i remember thinking was how much i liked it, the closeness of the experience, the excitement, it really didn't hurt at all other than the fear that someone would see me getting beat up.
I think that results, from having a somewhat abusive older brother.....to this day, i enjoy rough sex to a degree a fair bit larger than most. Not that i always need rough sex or anything but i am capable of dishing out and/or enjoying a fairly scary amount of violence as foreplay or part of the act. Of course there is all sorts of social and physical reasons as to why this isn't practical.
So yeah, that's my by no real means educated guess as to why you like biceps. Something good happened involving biceps and your body jumbled up the information around sex.
I don't think it's a coincidence your biceps are what you feel is lacking the most. You're likely to view other's biceps as so attractive and by comparison yours are likely to seem inferior.
I have to imagine that guy could curl atleast 180 for laughs. He's short too, so it'd be easier.