As the game changes, so too do the practical limitations of servers and clients.
LvD was halving the mass limits each reset for a while. The next iteration would have been a 250k mass limit.
..But this reset, we lifted the mass limits (for now). We'll be re-evaluating what the limits should practically be, but when we set limits it's not willy nilly. We WANT as much freedom as possible, but we also have to maintain actual fun PvP on our server. So we do a lot of testing before we settle on values that are equitable to run on a variety of PC's.
Typically the limits are not set because of server limits, but because of client limits. For example, one thing we noticed was that when a player ran a fleet of 5 ships at about 250k apiece (~1.2m), it would crash most people's clients as the ships moved in to attack and needed to all be loaded at once. Then the person being attacked by the fleet would get wrecked while they weren't able to pilot their own ships. This kept happening and was not fun for anybody, so we scaled the rules back a bit on fleets.
But really, I think the ideal is natural boundaries for ship mass. That is, there are practical considerations and vulnerabilities that make building a super large ship not worth it. For example, if effectiveness drops off and smaller ships have the capability of doing damage to a ship that is too large to maneuver. I'm curious to see what the weapons 3.0 changes bring to this, particularly the missile/missile combo which bypasses shields could be a giant game changer.