It’s great if you run max two “web apps” at a time. More, and it’s heading into “may be a problem” territory.
I’ve seen a Gmail tab eat 2.5Gb of memory all on its own… just sitting there. And you need some headroom for content and file caching and such to keep things feeling snappy.
cache uses whatever you have
im sure that gmail tab is just casually caching gigabytes of email, just in case the user opens it.