My little guy has doubled in size in a month. I am rusty and I cannot remember if my first dog did that when she was 4 months old. He was sick and immaciated when I got him. Between the meds and the food he is a happy little guy. I got him at 10.4 pounds. He has to be close to 20 now. Part of me was worried that he was over weight, but everyone I ask just laughs and says he is normal.
On a side note, he is so funny looking right now. His body has grown but his head has not, so he has this big body and this itty bitty head. And of course huge ears. It is an awkward stage but it is so FUNNY!