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.
Also:
http://www.gamespot.com/articles/gta-5s-online-mode-has-gene...
There is no denying Valve does great with the number of people it has, but of all the badly managed companies they could be compared to, Rockstar really it is not a good example...
In the last few years, Valve has developed multiple new pieces of hardware with software support; the Steam Controller, Steam Link, and HTC Vive. They've also developed and supported a new platform, SteamOS, and rolled out hardware with manufacturing partners. Besides this, the Steam platform has added live-streaming (like twitch), family sharing, big picture mode, the VR UI, and other features. They were also involved in the development of the graphics API Vulkan, which they need for their Linux-based SteamOS to become a better platform.
These are just what I can think of off the top of my head and don't even explicitly involve any games (though they released a collection of 11 small experiences for VR as well).
Interesting thought experiment: Break the sales for DOTA and TF2 into ~$50 increments and compare that number against total sales for the last two GTA's put together. I suspect Valve will beat them at least twice over.