bigger breeding tank better for cories?

What species of cory are you trying to breed? I am not especially knowledgeable about cories, but will offer my opinion. A 75 gallon is probably bigger than you would need, but it certainly can't hurt to use a tank that is larger than necessary. If you have the extra space then go for it - you could certainly keep a good sized breeding group in a tank that size.

Cories can be bred in as little as 5 gallons, its been done, but a 35-55 would suffice.