[SRD]

there is really no guarantee in weight until they reach adult age. I've had a girl who weighed 14oz at 12 weeks, we were so worried that she would not be able to tick the 2lb mark, but she ended up a lil over 3lbs, very chubby girl though. and my girl dolly on the other hand has done her growing during the younger puppy age. she just turned 3 and weighs about 2.5lbs.
lots of the tiny ones are just as healthy, but they still need extra care compare to standard sized ones, their bones are very fragile, their body temperature drop down fast during cold winter, their blood sugar level are harder to stablize thus have to be fed more often, the list goes on and on...