Jill,

For some comparison my son is almost 5 months old and weighs just about 20lbs and is 26 inches long.  He weighed almost 10lbs when he was born and was 22 inches.  Like you I was also concerned about his large size.  We saw his endo the other day and he said he was very healthy.  I find it helpful to plot his weight and height on a growth chart.  He started out in the 95th percentile and is growing on the same curve so the his ped endo does not think he is "overgrown".

Hope this helps!