There really is no way to tell, it is all a guess really. The double the weight at 12 weeks seems to be the closest thing but I have a guy that weighted 4+ lbs at 12 weeks and is 16 lbs now. My Chelsey who is two weighted 3+ lbs at 12 weeks and is 5.3 lbs now.
That being said our big 16 lb Teddy can fit in a bag and be really quiet and travels well. While Chelsey is a wiggler! Teddy is easier to take in a bag somewhere just heavier!
