(raises hand)
I fully understand the concepts presented by Duke and others here, but from the opposite point of view.
I'm an FA; it's an acronym for Fat Admirer. This means I am strongly attracted to larger ladies. (I try to refrain from using numbers to describe my tastes, but if you must know a range I consider attractive, start with Lindsay Hollister and stop with Darlene Cates.)
Like the OP, I could have a thin female friend, but it's very unlikely to advance beyond that. Thin women just don't "rev my engine", so to speak.
In my experience, personality and presentation can indeed be a factor in overall attractiveness. I've dated some BBWs ("big beautiful women") that, while not particularly beautiful in facial features or body shape, were fun-loving and enjoyed some of the same things I did. And to be sure, I met a few super-cute ones along the way that turned out to be real "witches with a capital B".
I consider myself lucky to have married a decently-proportioned, nice-looking woman with good levels of humor and common sense. If not, I would probably be stuck in the same situation as the OP, with everyone introducing me to their fat friend/sister/cousin/co-worker-- just because they know of my preference. Heck, the ones that know I'm married already say, "Hey, dude, I saw this chick at Wal-Mart the other day and immediately thought of you." If I were single, it would just get worse.