I have a black bantam OEG roo that sweet talks my golden star hen into squatting for him, then he hops on to do his thing and hops off. One of the funniest things I'd ever seen....
I have proof that my bantam OEG roo produced offspring with my black star hen. For a good year I had not had a standard size rooster around, my black star goes broody and hatches out chicks (only one managed to escape the fowl weather and the ants). The surviving chick is a pullet and is larger than my bantam OEG hens but not as large as the black star.
So, yes it is possible for a silkie roo to mate a standard size hen. It's just not likely to happen unless you help the situation or they eventually learn it on their own (like my bantam OEG).