I've had some EE medium sized bantams bigger than their fathers and mothers of different sizes.
This fall I am going to attempt to try Welsummer LF hens x Welsummer Bantam cockeral to get some medium sized girls and keep the darkest and breed down to carry the darkest egg color.
I was also curious about this topic. I have a few bantam hens and a standard roo. I've caught him trying yet he seems unsucessful but who knows. I think he's too heavy for the small hens. He is a spanish roo and weighs about 4lbs. The hens however are old english bantams no larger than a pigeon.
If it is a big roo on a little ( pigeon sized ) chicken then there is a good chance a hen is going to get hurt.
Chickens don't care if they're related or different breeds or sizes. They do not conform to human standards regarding reproduction.
They can and will breed. I have some medium feather footed blue and barred rocks to prove it. My step-dad's blue cochin bantam breed his barred rock hens and I hatched the eggs!
The size of the egg won't matter either. If it's a large roo over a small hen, the chick has just as much of a survival rate as a chick in a large egg.