It's a combination of game design and technology that determine if an MMO can have one "shard" or many.

Eve is able to spread their player base out over a vast area. I'm not sure how they built their "universe", but it could be pretty easy to have the programmers build a procedural world that the designers then go into and tweak here and there. The highly hand designed areas are the common areas players start in while learning the game, and the systems all have friendly names like "Hysera". The less designed systems with few to no NPCs can have friendly names, but many are things like "A-803L".

Eve is also a "sandbox" game (as was Galaxies), where players are given the ability to modify and change the game world. So even though A-803L may be filled with just random generated planets, players can take over the space with their corporation and claim it as their own. The players like having the control, and it reduces the work needed by the Eve designers in building out the world to make it fun to play in. Corporation wars are a major part of how people have fun in the game, and it's all "content" that players create themselves by playing the game and fighting others.

WoW, SW:ToR, Everquest and many others are more "theme park" games. The designers of the game create all the content, and the players simply consume the content. These games also take place mostly or entirely on the ground. Thus you need to build terrain, paths to travel, buildings and NPCs, vs Eve's open space. It takes a long time to actually build all this by hand, and the overall play areas are smaller. Due to the size, it's not possible to cram tens of thousands of players (and definitely not millions) onto one shard. If it was attempted, overcrowding would occur in the common areas, leading to a poor client experience (bad FPS, nothing to kill, can't find the NPCs to get quests, etc...), along with possible server performance issues.

Eve solves the overcrowding a bit in the common areas by simply not showing everyone else there in a graphical way. You dock at a star base, and there may be hundreds of there there too. They just simply populate a text list. And all the services at the station are simple UI interactions, vs moving an avatar around in a physical space. Out in real space, ships have collision (i.e. two ships can't occupy the same space), so it becomes hard to just stack tons of players into one area small enough to cause problems. WoW, SWToR and most other theme park games tend to not have player collision on, to avoid issues where people could turn themselves into baracades. There are known battles in Eve that have been so large, the server did have problems calculating combat damage, but their engineers continue to tweak and change the hardware and software to allow for larger battles.

Could Eve support a WoW sized subscriber list (~12 million) with their current setup? Thats hard to say. I can't find good numbers now, but Eve's total subscribers is somewhere around 350,000. Total number of players on at once has crossed 60,000 before. Most games average in the single thousands of players online at the same time per shard. Eve is also the oddball MMO in regards to the subscription and concurrent users graphs. It's pretty much a 45 degree angle of linear growth for both, over the games 8+ years of service. Most MMOs have a huge spike at launch, go down a bit, level off, then spike a bit again when new expansions are released.

Each MMO has a different setup, due to the tech still being considered "secret sauce". There are some prebuilt MMO technologies out there for use, but they are still quite young, and usually are still heavily modified to ship a game. Even if larger areas could be built, and players were not able to cluster together with enough numbers to bring down one area, the database or other pieces of tech may be responsible for the players per shard limits.


Oh, one correction on SW:ToR, I did notice last night that there did seem to be a second instance of the common play area I was in up and running. So there is some vertical stacking of zones as needed, but the limits here seem low based on the queues to log in. Odds are, the server queues are high even though the overall SW:ToR play area is mostly empty due to everyone overcrowding the newbie zones. I know WoW runs like this as well, as a LAN party I attended years back was able to trigger the login queue simply because we all created new characters in one starting zone. The queue was brief, but shows that Blizzard has some sort of automated process in place to throw up a queue if any one area of the game world becomes overloaded.