They figured out a way to code the servers so you could get huge servers in BF2, PR and FH2 mods (saw some 200 man ones that were full). Now they know how to do it, they might be able to do the same to BF3 if they haven't increased the sizes. Then of course it's simply a matter of renting servers to handle the load, seeing as the only real problem with those servers was the horrendous lag because they weren't capable of handling the numbers. The way they did it as well didn't breach any terms of use that Battlefield had.