I have a bull that throws nice small calves and have never had a small heifer having problems out of 70 or 80 calves. Problem is he is now full size and 6 or 7 years old and I have seen him squash a 14 mo heifer onto the ground trying to mount her.
Year before last I had two heifers I had to cull from injured hips from bulls being too big for them. I went and got some young low BW yearling bulls to put over my yearling heifers last year.
So not only do they have to throw low birth weight calves - they need to be a reasonable size compared to the heifers you are going to put them over.
I have read that heifers run around a lot and that big heavy old bulls can have trouble keeping up with them. It sounds a reasonable proposition but I dont know from experience if it matters.
Saying that, my older bulls have got 4 - 5 mo heifer calves on their mothers in calf.
You might get away with it with only a few heifers that are strong and IF the bull hops on them just right. You are not going to know if he is going to squish them flat until after he has done it. It is all very well to say some bulls might be big but will mount a cow in a light way - but you cant know if he is one of them before hand.