Earlier quoted context omitted.
Rockstar North's facility in Edinburgh has roughly the same employee count (slightly more by Wikipedia's count), and all they do is put out one new game every 5 years or so. All they did was recreate a city full of assets and added game mechanics to it in five years and made the most expensive (at the time) game product in history with more than 1000 people involved in production and the fastest selling entertainment…
What's your point? Valve put out multiple AAA games in the same timeframe too, and did a lot of other stuff besides. Sure, GTA5 is a big-ass game, but Valve's games - games plural - haven't exactly been small projects either.
Rockstar has an army of artists working on those assets for that game. Valve is a multidisciplinary house where they dabble in a lot of stuff, split in smaller groups, but their primary vehicle is Steam and a couple of (successful) games where each project does not need as many people. It's comparing apples to oranges.
What Rockstar does demands an army of artists. If Valve were to do same type of game world, they would have an army also. They do not do the same thing though.
If Rockstar were to do what Valve does, they would also have a number of people split into smaller teams working on their own things. They do not do the same thing though.
That's why you can't say it's all Rockstar does with same number of people. Those people are not the same skill and those projects are not the same.
Rockstar's yield, if you're comparing the game industry, is a level above Valve. It's a level above most. It's really not a good example to compare to. They are on the leading edge of what they do. An area Valve isn't in. Maybe if you've compared Avalanche Studios and Just Cause 3. They had 75 core-member team in a studio of 250 and, who knows how many, outsourced people.