Take it from a fat man (me). Don't look a gift horse in the mouth man, please!!
You can't control how much fat you add to your body. It'll be a vicious cycle of gaining too much, then working hard to get it down to what you want, to gaining to much, etc... etc.. etc. And in the long run it really sucks.
When I was younger I was built pretty well, played Hurling 3 hours a day 5 days a week and looking back at myself then, had an excellant body and fitness regime. Then I got caught up in life. Drove instead of walked, ate, ate, and ate some more. Worked 80+ hrs a week, and now I'm over 300lbs and I never even saw it happen. One day I saw a photo of me and my nephew and realized just how farking large I was, and I damn near broke down a cried.
Don't do it man, please don't, you'll hate yourself in the future.
Add some more muscle in the areas you think are to small. Check this out:
http://www.exrx.net/Lists/WtMale.html
Just click on a body part and it'll show you several ways of adding muscle there.
Be happy with what you have, because the older you get, the harder it is to keep your body in shape, and if you purposely let yourself go now, even just a little, you'll be killing yourself down the road.