It's not a weight thing, It's a height thing.
I just posted this in another thread but you may find it useful. Follow their instructions to find out your proportions and use it to see how far off the bike size actually is.

http://www.competitivecyclist.com/za...LCULATOR_INTRO

If you're just a little bit off your "ideal" size some minor adjustments could be all it takes for you to be comfortable.
If you're considerably off in size then you can worry about switching things out or whatever it else it may take to get things right.