
If you have a very large rooster like a Jersey Giant paired with very small bantams like Sebrights, you might see some injuries during mating since he will be so large and heavy for her to bear. However, the reverse is also true: a very small bantam rooster will have to be fairly brutal to be successful in mating a large hen, and will often injure her comb as he holds on for dear life.
To minimize the problem, you may want to get a smallish, non-bantam rooster for your mixed-size flock, like a Hamburg, Leghorn, Polish, Dominique, Easter Egger, Campine or Andalusian.