By bigger, we're referring to larger maps and more cities to visit in MMORPGs. While a lot of people will probably have a knee jerk reaction to this and say "yes, it is" there are a couple of things we have to keep in mind.
If you're a veteran of MMORPGs, I'd like you to recall the instances when you have to travel long distances in order to complete a quest. Now, this is great for gamers who actually enjoy exploring different regions in the game. However, even to this specific group of gamers, it can get old really fast especially if the landscape is pretty much the same all throughout without any notable landmarks. In this case, bigger is not better.
Another thing you might want to think about are the problems associated with having to kill various enemies over large empty spaces. Some MMORPGs are so massive that you will kill a mob and have to travel some distance to find the next one. If you've ever experienced that then you'll probably be thinking, bigger is not better.