Weird, Oldry sent 400 subspacers against one of my planets, and had a 14.9% chance of capturing my planet (I forgot to build a scout one turn). In other news, Airstrip 3 should have PD in 18 turns. Just 18 more turns of scouts and blackbirds. Then mass death of Oldry's subspacers.
Which, to me, says, "size doesn't matter (much)." Because that's a big fleet and still a fairly low chance.