Your baby takes from you so of she is small then she may not be getting all the vitamins and minerals she needs. If you don't like your prenatal go look for a really good multi vitamin that you can take while pregnant. You gaining weight and eating tons doesn't mean she'll gain weight or get the nutrients needed unless you eat balanced meals.
I eat a lot though it's just hard for me to gain any weight
When my mom was pregnant with my sister she measured small her whole pregnancy they just told her to eat more bc she wasn't eating enough and my sister is perfectly fine