A full screen at 4K is 8 megapixels or 32MB or so. I guess if they double or triple buffer each app might take 100MB or so, but you'd still have to open 10 apps to reach 1 GB.
10 app windows doesn't sound like a lot to me.
Besides that, if you enable resolution scaling macOS renders at a higher internal resolution and lets the compositor scale it down. Try that on a 5k screen...
I mean, what of it? 10 windows doesn’t seem like many, at all. I tend to end up with at least ten terminal windows by the end of he day, nevermind everything else.
