Ask Massively: This is why I will get eaten by wolves edition

It depends a lot on the game and the specifics of what the designers want to do with it. Smaller sizes are by no means a requirement even in newer games -- Final Fantasy XIV boasts parties that can swell to 15 members, for instance -- but there has been a trend to shrink maximum sizes down in recent years. The reasoning behind it is all down to what designers want to do with a game and what they hope happens with groups.

Smaller groups do two things: They diminish mandatory player roles and they accelerate the formation of groups. It's much easier to get three other people together than five, and it's much easier to look for a tank and a healer than a tank, healer, support, and debuffer. The upside to all this is that players have a much easier time getting into a group and tackling harder content. It also emphasizes individual performance -- one bad player in a group of 4 is much more significant than in a group of 10.

Global Agenda, DC Universe Online, and Vindictus all have a very strong action/skill bent to them, hence the trimming of the party down. There are also systems in place to de-emphasize certain roles within the party, since everyone is responsible for damage in some respect. By contrast, RIFT has a more traditional game setup, so a larger party size works well with it and allows for more nuanced roles. And while several newer games have gone for smaller groups, older games that are still quite alive have significantly larger ones, with EVE Online boasting an enormous maximum size.

Lastly but certainly not least, it's partly in response to the fact that older games had much larger group sizes. Considering the usual trending of video games, don't be surprised if in a few years group size is back on the rise.

Considering how you beat World of Warcraft, I'm going to suggest Final Fantasy XI. The deletion dialogues are legendary, and because of the way the game is structured you'll be able to beat it even faster.

In all seriousness, it depends on what sort of game you're looking for. Lord of the Rings Online is a fine alternative with a more roleplaying-heavy bent. Warhammer Online is another easy step away with a much stronger PvP focus. And Guild Wars is also a fine option -- it's free once you get past the box price, which means that there's a lot of fun to be had without worrying about a fee. That barely scratches the surface of the options out there, so I'd advise a bit of research and figuring out exactly what you want from a game.