Zipper Interactive have revealed the reason they chose 256 as the number of online gamers able to play MAG in one match. In an interview with German site Play3.de, a Zipper Interactive developer said (not the best translation by Google);
"We have chosen deliberately so for 256 players. Not because we are a technical limit prescribing. Certainly we would be able to tickle a few more pieces from the hardware, but 256 players are perfect to give you the feeling of an epic battle. With a higher number of players would lose the individual and may no longer be seen as part of a team."
It was last month we found out that the game could support more than 256 players. But it seems the reasoning is for a more realistic feel to battle (from a game based in 2025).
The game is out exclusively for PlayStation 3 on January 26 in the US, and January 27 in Europe. You can try out the public beta of the game by downloading it off the PlayStation Store (make sure you have enough room on your hard drive).
http://connectedconsoles.com/ps3-Zipper-256-Perfect-Number-For-Epic-Battle-Feel.cfm