I just had to go back and check out my kids weight records...
DS#1 was 17 lbs 5 1/2 oz and 27 1/2 inches long at 6 months...for some reason his 4 month visit isn't in here
DS#2 was almost 16 lbs and 25 inches at 4 months. He is now 10 1/2 months and over 22 lbs. He is HUGE. People tell me all the time how big he is. And up until just recently he has been pretty much exclusively breastfed. He has roll upon roll, and I love it.
Try not to worry too much about weight at this age. Kids that start out eating healthy (breastfed babies) and whose parents don't feed them a ton of juice, soda, candy and fast food are quite healthy and not obese. It is all in the diet, not because they were chubby babies.
It kills me to see babies in the mall eating french fries...and we are talking babies. Under 1. Yuck!