I keep hearing this, but it's actually BIGGER than many think. It's 15 people, half of whom are directly working on material. Compare that to most RPG companies, and you will find it's HUGE. People keep comparing it to Paizo, but that's not the only comparison one can make. Compare it to, say, Green Ronin, and you will find it's massive. Compare it to Monte Cooks company, and it's really quite big. Heck compare it to 13th Age and I think it's huge. 15 person teams are quite large for the RPG industry in general. I think it might be the second largest RPG team right now.
There are several companies with relatively large teams, tho' not all team members are full-timers.
FFG appears to have more than 15 people... they've got about 20 in the credits amonst the 9+ lines (EotE, AoR, F&D, DH, RT, OW, BC, DW, End of the World) being supported (in 3 engines - Star Wars, 40K, and their new end of the world line).
AEG has a sizable team (tho' I've not checked the current edition) for L5R. Not certain how many are full time, but it's a long list. Looking on RPGG's entry, 15 production staff, 4 designers, for L5R 4E core (tho' that was a few years ago; I'm not checking supplements).
Mongoose has about 5-6 in the office (not all full time, I suspect), plus a bunch of recurring freelancers.
Palladium has a stable of a dozen freelancers that Kevin rewrites and then gripes about... but he still pays them...
So, Wizards isn't unique in a large team. They are unusual amongst the bigger teams in having them all in-office and apparently all full-time. But, by the same token, WotC's D&D side isn't a pure RPG team, either. They are a pure D&D team, but they work on the RPG and the related board games.