Six of one half dozen of the other. Some bantam breeds are larger than others making mating easier either way. A serama roo for example will not like 99% be able to breed a standard hen even on the smaller side and a giant cochin will not be able to cross on say a dutch banty. It depends on the breeds in general... A silkie roo may however be able to reach say an australorp hen... and a leghorn male should technically be able to reach a bantam cochin hen.