The R-Types aren't that big; certainly smaller and would produce less drag then the others. Even if the Ogdoad is focusing on them, with Ebon's undivided attention on the R-Types themselves they're still fairly agile, even underwater. 

The reason I'm being nitpicky about this is because I don't want people to think that just because he can swarm most people with superior numbers that he's just going to wastefully throw them all away. Having a hundred R-Types doesn't mean anything if they're just heading straight for the grinder.