I agree with what was said here and I have many years of experience with roosters. My thoughts I'd like to add are these:
On current size, how they're both Silkies (if they were bred pure), they most likely will be about the same size once fully mature. The bigger one is the more dominant rooster and is why he's appearing bigger. This is also why he's the first to crow and he'll be the top of the pecking order between the two, having the final say on who gets the ladies. Depending on the hen's themselves, if they had to choose, they most likely will choose him over the other rooster, as they'll pick the more dominant rooster.
On matured size, in my opinion, it does matter, but them both being Silkies, they shouldn't have enough of a size difference to matter. And what I'm meaning here, is if you have smaller hens (according to the rooster's breed), you want a small rooster as a bigger rooster could accidentally hurt your hens. (A quick example of this is, if you had all Silkie hens, you wouldn't want an Orpington rooster because the Orpington rooster would obviously be too much for such a small hen.)
On size to ratio, size will also matter. A smaller rooster needs more hens. A bigger can go with less. These roosters being Silkies, they could possibly go with more than twelve hens each, but how much they can see to catch the ladies will probably make up for that. I had a Silkie rooster in with about twelve Standard hens, and they were plenty enough for him.
In the end, these both being Silkies, I think your best option is to follow what the above poster said. This is going to be the rooster of your flock, so you want the one with the best temperament with both the hens, and you. And I'm saying you because I've seen plenty of aggressive Silkie roosters. You don't want a rooster that is your hens' every dream if its attacking you and any small child that comes around.