You know i just thought about it and with my bigger dogs the the doubling at 6 months rule was right by pups doubled their weight at six months plus some but then my minpin a toy breed was at 6 1/2 pounds at 6 months and she was full grown at 7 1/4-7 1/2 pounds. Of course after a few years she got overweight on me but i am happy to say now that she is almost back to her healthy weight
So yeah i have to agree somewhat the 6 month rule does apply to some but i think it is the bigger breed that it applies to