Well, in my case, it would be a bad idea. My Delawares are substantially larger than my bantam Brahmas and they would terrorize them. I once had an escaped bantam male chased into a ditch by my Del roo, and he was trembling with terror when I extracted him and put him back in his nice, safe pen. As it is, my Del roosters stalk around the perimeter of the bantam pens, but they can't do any harm.
I know that bantams raised with standards can fare better, but in my experience, even that means that the banties can get bullied some.
Bantams on range may be easy prey for hawks, who don't always see large standard chickens as prey (at least not right away). As a general rule of thumb, it's better to pen up the bantams.