Well, since you asked nicely, let's do an estimate (for 2017):
Microsoft Gaming revenue: 9,25 bn (confirmed from annual report)
Valve Gaming revenue from game sales only: 4,3 bn (estimated by SteamSpy)
Now we don't know how high a percentage of Valve's revenue is made up from other sources than game sales (mainly DLC, microtransactions and the marketplace). But we can look at another major western game company: Activision Blizzard. They revealed in their annual report for 2019 how much of their revenue came from "in-game net bookings" (fancy talk for DLC and microtransactions). They revealed it for 2019 (around 53 % of their revenue) and for 2018 (around 57 % of their revenue), so around 55 % on average. And the remaining 45 % aren't even only from game sales, since they write in the report that it also includes merchandise and license fees.
If we just keep it at 45 % of the total revenue and say theoretically that Valve was in a similar situation they'd have a total revenue of 9,5 bn in 2017.
This is just hypothetical of course and you might say that Activision Blizzard games are riddled with microtransactions and the works, so they're likely to have a bigger part of their revenue from those sources than Valve. But many major games on Steam are free-to-play and make all their money from microtransactions including what are probably the most profitable games for Valve: Dota 2 and CS:GO. On top of that there is the marketplace where Valve makes money from players reselling items to each other, which Activision Blizzard does not have an equivalent to, and I don't think the significance of that should be underestimated. You may still not be convinced that Valve's revenue is bigger, but I hope this is enough to at least show that they are in the same ballpark.
I'm sure if we looked at profit than revenue then Steam certainly have higher profit.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"
Azzanation: "PS5 wouldn't sold out at launch without scalpers."