Listen, if you will, and perhaps we can uncover an intersting answer, or at least an enlightened perspective.
There are a few things that I have taken for granted, things that, upon examination, leave me confused. Now I know that the population on many FFXIV servers hovers "dangerously" around the 700-800 range, and I know that many would consider it, currently, a relatively "low population game." I also know that WoW is considered to be the highest population game of any.
That said, I have been recently playing around with my friend's WoW account, specifically with the popular and reputable WoW census mod (more than on any character, humorously). Now this mod, which has been around for many years and still updates very regularly, continuously reveals the population of just about any server on which I play to be in the low hundreds -- I mean 250-350 most times of the day -- it checks all characters in all zones of all levels in your respective faction, so the population of the server itself is likely to be about double whatever number it returns.
I know it to be accurate because I can do "/who" searches manually (or just look around me at the phenomenally barren, non-hub city zones across which I am lucky to encounter any other living player, even for a brief second, or of the opposite faction) and turn up similar results. Now I'm not sure how many servers WoW has, but for it to have as many players as it claims it needs to have at least a few hundred; I am not necessarily saying that they lie about their numbers (although I would wager that every MMO inflates their population, given that it is such a huge conflict of interest), but I am saying that I was underwhelmed at the isolated feeling only a few hours tinkering around left me (I played my friend's reasonably high-level character and started my own).
I suppose I am merely saying how ironic I found it to be that a game that has 50,000 players seemingly allows me the potential to interact with more denizens of my server than one that has 14,000,000. I guess it just taught me that numbers, though important for longevity's sake, are not everything.
Make of it what you will, though: these are simply thoughts, and I am not passing judgment.