7 pounds is still small tho! I wouldn't worry to much about it. Diesel was the runt of his litter. His mom was 5.5 (while preg.) his daddy was only 4 pounds. She only had 2 babies and Diesel's brother was right at 2 pounds at 9 weeks. Diesel was only 1.8 pounds at 9 weeks. Now at 18 months Diesel is only about 5 pounds.
It could be a growth spurt or there just might be more of him to love. Who could argue with that! The more Yorkie the better! LOL