you can actually buy weight-gain powder stuff from boots, if that's any use.
other than that, no you're not the only person who feels that way about summer! some people like to be frazzled, some people prefer it when it's cold - there is nothing freakish about it at all. there are plenty of people out there who don't have the guts to walk around half-nekked just because it's sunny, so don't stress about it. i'm average-sized but i still hate poncing around in teeny-weeny clothes and i think that bikinis are actually quite evil. you're really not alone with the confidence issues!
i know it's easy to feel down when everyone else seems to be having a better time than you; but just stop comparing yourself to them. everyone's different and that is a GOOD thing. your friends value you for who you are, and i bet they aren't trying to make you change! just relax and enjoy the fact that the summer solstice has gone by, so we are now on the road to winter, scarves, coats and jumpers!