Will bantams and full-sized chickens crossbreed if the flocks can intermingle?

[shinysatins]

If a flock of bantams and a flock of Barred Plymouth Rocks are kept together or free-range together, will they tend to breed within their own size, or will they crossbreed?

 

[goldfinches]

I've asked this question about my seramas and full sized roosters and been told they'll breed. I've also seen them doing it...

Never hatched an egg to try it out, though.