Your number equivalency had me chuckle.
Bungie is a small player in the market. 4% of the market cap of Activision, which has a higher market cap than Nintendo and almost as large as the entirety of Sony. It's understandable why the largest tech acquisition in history that's buying out the largest IP and revenue base in gaming, would be a much bigger deal and worthy of at a bare minimum contractual stipulations to guarantee free market activity that doesn't cause long term harm to the competitiveness of the industry
This has nothing to do with how big Activision, a gaming company, is in relation to Microsoft as a whole, a nearly $3T windows, office, and cloud supplier. We are discussing two very different markets
So yes. The scale of the purchase matters. Bethesda, despite being a large acquisition (largest in gaming history at the time), was not under regulatory scrutiny; it wasn't big enough to be anticompetitive - no deals were necessary to ensure competitive markets in that case.
And yet, Activision is 10x the size of the previously largest acquisition in gaming history. Of course the situation is different and warrants active regulatory intervention.