Yeah, what everyone else said. I'm at 330, and I train with everyone else and can do what everyone else does. I get winded more quickly than some of my skinnier classmates, but time (nearly four years now) and experience have show me how I can use my size to advantage.
The key thing to understand is that skinny people have their own physical challenges when learning aikido. We have the whole range of body types in my dojo. There are a couple of people who can't weigh more than 120 or so. There's one guy who works out a lot and has a pretty muscular frame. There's me, the big guy, and a couple of guys in between.
They (we) all struggle with learning aikido! We are also all learning it pretty well. I've noticed that the smaller guys in my dojo have a hard time, at first anyway, being stable and projecting strongly -- something we big guys find easier to do.
Anyway, don't worry about it.