Both my labs are small as well - 54 and 57#. Not sure that it is a component. A lot of the girls at the kennel I get my dogs from are all small (under 60#) not sure that there has been a particular problem with heat cycles and body size. Good luck with your girl.
P.S. thanks for the compliment on my girl - I of course think she is gorgeous as well. I am not biased or anything.
Dr. Brown -- Thank you very much. I guess I am going to hang in there. There is really nothing else I can do. The only comment I can make is that some of the responders to my question also seem to have or have had very tiny Labs. At 51# as of 2 days ago thats small. Although she did gain 1 # in a month. Late last year she started out at 49. Thank you very much for your input and comments. By the way your lab in the picture is beautiful. What a coat... Jeff