I breed labbies. On the size issue, all I can say is 'patience'. 19 kg isn't that light for six months, so I would guess that she's got some bone, which is good. And while I can't condone the breeder putting you off, 6 months is an awful age to evaluate. The imported dog I'm now hoping will come out as a good stud dog started growing rapidly in the bum at that age and his front legs didn't catch up until nine or ten months -- with the result that he looked awful. He's looking pretty good at 10 months.
For all I can tell, the jury is out on the food question, apart from 'don't supplement calcium' and 'don't overfeed'. Everyone has opinions and experiences, but they point in many different directions.
Big can be worse than small. The most fantastic looking litter I have had (bitch on breeder's contract, the owner feeds a strict BARF diet) filled out beautifully in their first nine months, all large and all well proportioned. Three of them have been hip/elbow scored. One had 3:3 elbows and at two and a half is limping, one had a hip that was so bad that the owner didn't get scoring done (both were castrated), and one had 1:1 hips but 0:2 (2 mm) elbows . . . she does beautifully in the show ring. Their lines have elbow problems (ie, scores of 2 on one side) here and there, but none of the immediate ancestors had problematic.
The same bitch was put to a different dog and most of the pups came out slight (fine bone) and some of them short as well. None made it to breeding quality.
If I were the breeder, the best way to approach me on this would be to ask for shift to main register so you can show, while offering to sign a Non-Breeding Contract that says you cannot breed for your bitch until such time as the breeder agrees she is of breeding quality. If you do well in shows, the breeder should come around in time and -- if you want to -- agree to let you breed from her. Most of us are proud to have winners out under our prefixes.
Others may not agree, but I think it would be better if Limited Register allowed showing up to one year of age . . . that would give everyone a chance to get pups evaluated via the showring, and give breeders a better basis for deciding which pups to promote from Limited to Main.