Hi, we were giving my son a bottle during the night right up until he was about 12 months old. I think it was more a comfort thing and when we decided at 12 months to drop it we didn't have a problem. We started by making it very weak formula and then got to the point where it was really just formula flavoured water. He eventually decided it wan't worth waking up for and stopped waking up entirely.
I think that your daughter will drop it when she is ready, but if you really want her off it, then either go cold turkey or wean her off it gradually.
And as for her not putting on weight very quickly, maybe she is just going to be a smaller child? Some kids are bigger than others. Yours may just be delicate and dainty!