One thing to keep in mind - the 'average' figures are just that and are based on data from millions of babies, so remember that there are lots that are on either end of the spectrum. Its true, you really can't overfeed a baby.
Having said that, my friend's little boy (currently almost 8 months old) was 7lbs and change when born and almost 20lbs by 4 months old. Yes! He's a happy and healthy little boy though, he just got a lot of his growth spurts up front. = ) My newest DD, now almost 7 months old, was 8lbs 2oz when born, lost weight and now weighs over 19lbs! Since all the books say that the 'average' baby will be around 20-22lbs at 1 year, both of our kids are well ahead of the 'average'. So don't fret, it will all even out over time.
My first DD was so chubby as an infant I thought she was going to pop some days! In the 90-98 percentile for weight (but not for height, she's short like me), I was worried that she was 'too fat'. The doctor told me not to even concern myself with things like that until after age one. And now, at 4 years old she is stocky and sturdy, but actually quite slender, so it all works out. (The biggest change in her weight started once she was walking/running on her own.)
So, I would say, enjoy your chubby baby - I say a fat baby is a happy baby! (and a happy momma as he starts to sleep even longer periods through the night.)
Good luck!