I've made 134,000 gold since early November (I got MoP in mid-December), basically with one character on a medium-to-low-population realm. (Like, it started off as medium, and now it's low.) So it can be done, though it might be time-consuming.
Finally, is it better to have a big economy or a small one? I naturally assumed a big one, because you have more buyers and more stuff to buy, but on a small one you have less competition and can probably list things at higher prices.
I think it depends on whether you want to sell ordinary or specialty items. With ordinary items, you're probably better off on a lower-population realm, because there are fewer people undercutting you, so you can spend more time as the lowest seller. I sell a lot of glyphs this way.
For speciality items, you want a bigger server, because those items have fewer buyers. I have an advantage with glyph-selling, but I have trouble finding people to buy my 20-25,000 gold Darkmoon relics.
Another thing to consider is that selling ordinary stuff is a lot more time-consuming. It doesn't take much time every day to make a Darkmoon card, but canceling and re-posting all your glyph auctions can be tedious.