Sorry, I am guilty of being the one who probably took it way off topic....
Let me ask a few questions...Have you any experience with cattle? If the heifer was tiny at birth, then it often turns out that they just don't get as big as they might if they were normal size. Or they take an extra year or so to "catch up". Ask you vet if they can test her for a dwarfism gene....Sometimes we have found that these little tiny calves have problems that you might never see but there is always the possibility that there is a genetic reason. And sad to say, often these small calves don't breed because nature is saying " you can't have a baby and so shouldn't get pregnant". The problem with breeding these little tiny animals that are not truly genetically miniatures, is you are perpetuating a defect.
What breed is she and how big is she at 6 months? If you have done all the things you should, give her til at least a year. I would also make sure she is getting enough protein in her feed, pasture is NOT ENOUGH for a bottle baby that is weaned at 8-12 weeks. They need more than most people realize. If you look at a typical beef cow; she has her calf on her for approx 6-9 months. That calf is still getting milk as well as learning to eat feed and/or hay/and/or grass. Their rumen has been developing for several months to handle the roughage. But they are still getting the protein and fat from the milk and their body is "growing" on that. A beef cows' milk runs in the 4-7% butterfat range as well as having all the other nutrients they need. And they are getting it whenever they want.
So we go from there to the typical bottle baby. Say a normal dairy...calf gets colostrum, then is switched to milk replacer. First off, it has got to be all milk, milk replacer. The soy crap doesn't cut it. I don't know a single dairy that I test at that uses soy because they don't grow on it. Then you are giving them a 4 qt bottle 2x a day and in a couple weeks the calf is introduced to grain and then some hay. Many dairies have gone to 6 qts of milk once the calf is doing good and not scouring, instead of 4 qts a feeding. Milk gets tapered off at 6-12 weeks; when the calf is eating real good. Dairies keep that concentrated feed up until the heifers are in the neighborhood of 15 months and they are breeding them. They might get taken off "feed" and put on pasture until they come into the barn fresh but usually they get a supplement all the time. Right or wrong, dairy animals have been bred to need the extra nutrients to grow. If you want them to grow, they need the feed. There is no cheap way to raise a decent dairy heifer if you want her to live up to her genetic potential. Once she is mature, you can cut the protein back but during the growing years she has to have it. Maybe she was tiny was because she was premature? Or maybe the cow didn't get enough nutrients while pregnant and it resulted in a smaller calf. And sometimes it just happens.
Any chance she was a twin? They are often smaller... and will take longer to grow out. If she was a twin to a bull, there is an 90 % chance she will be a non-breeder. When a cow gets pregnant with twins and one is a bull, one a heifer, the male hormones develop quicker and often suppress the heifers hormones. More testosterone than estrogen in the placenta....The females often have incomplete reproductive tracts. There's more to it but I won't go into it now.
I am sorry, I tend to go on a bit. I learned the hard way MANY years ago, although there was some cattle history in my past...but i really don't want others to go through the same hard learning curve if I can help. Ask away...someone will always try to help with what they find does/doesn't work.