Yes they can, and will, and do, and I have their MEDIUM-SIZED offspring to prove it!
I would think their accuracy rate would be better if the hen & roo are more of a match for size. And a large roo might be too heavy or rough for a really delicate little hen. A little roo can mate with a large hen, but he will need the hen to be really compliant & patient.
When chickens mate the hen will crouch on the ground and lift her tail. The roo stands on her back and only needs to get his cloaca to meet the hen's cloaca for what seems like an instant. Nothing is inserted.
It can be hilarious to watch. Once I saw a bantam roo approach a standard sized hen. She was a willing partner, crouching down with her tail up. But the roo was inept, grabbing the hen by the back of her neck with his beak, and scrooching his bottom backwards trying to meet up with her cloaca. Only he was too short to make his tail-parts touch hers. The minutes ticked past while he continued to hold on with his beak and wriggled his butt around. Finally, the hen had enough and stood up, with the roo still hanging on. They looked like a rodeo act, the hen turning around & around trying to shake the roo off her back, and him hanging on tight, flapping his wings for balance. Ride 'em, cowboy!