[lil fu fu girl]

Your frame size is determined by genetics.
The amount of meat on that frame is usually determined by the nutritional intake.
If you are feeding a puppy, you should feed 3 times a day. Their systems require food for growth as well as for maintenance.

The long discussion of wether your puppy should have fat stores or not will rage on and on. I prefer to have my puppy with a little extra weight just to ere on the side of caution. If he has some fat, then hypoglycemia will not pose much of a threat. From what you are saying, he sounds just fine to me.
Ignore the Village Idiots out there with their comments about his weight.