are cockerals too big

My sister has 2 large cockerals, they are some German breed that are breed for meat.
She has 13 hens but are smaller mixed breeds.  some of her hens are getting bold patches on their backs which we know are due to the cockerals mounting them.
my sister thinks its due to there size but i thought maybe it was due to them being young, abit clumsey and very active.
she has seperated the cockerals from the hens for now but has become attached to them and wants to keep them. (the cockerals get on well together and she dosn't want to seperate them from each other)
Advice please on anything todo with the above

Once a hen is mated she will remain fertile for up to two weeks. If you would put the cockerels in with the hens for one day per week that would allow the hens to rest and heal up between breedings.