I bred rats in the past and it is not the 'done' thing to cull because the rat fancy is very pet orientated. People who cull are seen as cruel and are totally ostracised within the fancy. It's very different to the mouse fancy.
I cull mice mainly because I couldn't possibly find enough homes for them. No one wants mice and timewasters and collectors make up a good 95% of the enquiries I get, so I don't sell pet mice at all any more. I never had a problem finding new owners for the rats I bred as they are very popular pets. In addition, male rats are happy living in a big squishy group, whereas male mice are not. I had one cage for male rats, it was huge and had about 15 in it. Now, I need seperate cages for each breeding male mouse.
It helps to cull mice if you're showing in the under eight weeks classes because the remaining kittens are much bigger and stronger, but the adult size of mice is genetically determined - so providing you feed a good diet, the mice with 12 brothers and sisters will eventually catch up with the mice with only four siblings.