Gidget was 2 oz at birth and at 8 days she was exactly 5 oz, but she did gain a little bit every day. She is 10 months now and still small at 3 lbs 12 oz but I can see she has a lot more filling out to do and should wind up over 4 lbs. Her brother, Sebi, was bigger than Gidget at birth and was 5.6 ounces at 8 days. He's actually smaller than Gidget in size but not nearly as thin. At week three they both really took off and were gaining well every day.
Good luck with your little baby.