Lucy initially lost a few ounces when I got her - and that worried me. At 6 months she weighs 5lb 6oz. My breeder expects her to top off at around 6 - 6.5 lbs.
Also, I think when you're dealing with such low weight to begin with, using different scales may make a difference too. When I first picked up Lucy, I was told she was 5 lbs. When I went to the vet, she was 4.5 lbs. When I weighed her in my home, she was 4lbs 10 oz.
Now, I'm just going by my scale. As long as she's gaining, then she's eating and I'm not worried.
I met a girl recently who thought she bought a 'tiny' and ended up with a stocky little yorkie that weighs almost 8 lbs. Unless your breeder has a really complete history/pedigree of your pup, then it's difficult to even guess at the final weight.
Best wishes,
Tracey