Ashababy, I think its very hard to tell completely until they are a year old. My puppy had huge growth spurts, but she's not fat at all, but she is 7 lbs now at just about 15 weeks. But she's stayed the same for a few weeks now. She's probably going to grow some more, but as long as she's growing properly for her particular size and is healthy, thats what matters the most. Im so comfortable with her size right now and wish she will stay this size. So easy to pick up, but not so tiny that I have to worry as much about her getting hurt accidentally.
But I do melt at the sight of grown 5 lb yorkies, lol.