If you're wanting larger Buckeyes you just need to find some from a good breeder. Mixing anything with your buckeyes to make them bigger is only gonna leave you with something that is not a true buckeye. Besides, it'd take forever to breed out what ever you add to the buckeyes.
If you can't find any bigger ones, just start culling hard for weight like everyone said earlier.
If you can't find any bigger ones, just start culling hard for weight like everyone said earlier.