What breeds?

[AOrchard]

Is a small rooster likely to mate with hens twice their size?

Oh yes. Nothing stops them trying.

 

[NatJ]

Is a small rooster likely to mate with hens twice their size?

Yes.

 

[NatJ]

These are assorted from TSC, females. They're about 11 weeks old.
The silkie is an easy one.
The little black cotton ball has frizz with curls at the tips. They were all sexed but her comb seems pretty red already.

TSC had the labels wrong.

The Silkie and the Frizzled Cochin were straight run, because they are bantams.

Sexed bantam chicks are only sold by a very few places, and the prices tend to be very high ($10 or more per chick).

I would be very surprised to find sexed bantams at TSC, but not at all surprised to find wrong labels.