There's the practical matter of whether a bantam hen gets squished by a large rooster during mating, or whether a bantam rooster can get himself lined up right with a big hen, but once you have fertile eggs they should hatch just fine.
Some bantam chicks are known for hatching earlier than large-breed chicks, and I think that's due to the egg size. So crossed chicks in small eggs might hatch sooner than crossed chicks in large eggs.