My huge production red cockeral keeps breeding my leghorn hen, and she only weighs about 2-3 pounds (a runt from the feedstore) so is it ok, or will it hurt her? The production red weighs about 6-7 pounds (maybe even eight)
I don't know. But, I would watch for feather loss on her back and check under her wings for wounds from his nails. I am currently working on my sewing machine to get ready to make aprons in case I need them. I check my girls often for feather loss or wounds. I am lucky in that my rooster seems to be pretty darn gentle with them.
Practically all roosters are significantly larger than the hens of the same breed. If you notice, when the hen squats, she basically lays on the ground. That spreads the roosters weight out so she really doesn't have to support all that weight on her legs or with her muscles or bones. It is the same principle as a few pounds of pressure in a car or truck tire can support a whole lot of weight. It is possible he could hurt her if his technique is rough or if she resists a lot, but if the mating goes as it should, probably not.
I think one of the factors that can help cause barebacked hens is the difference in the size of the rooster and hen, but there are a lot of different factors involved in causing barebacked hens. I'd certainly be on the lookout for that kind of problem, but you are probably watching them anyway. I would not be overly concerned.