From my experience dealing with the extremly big meshes can be very different depending on what you are accually doing with them. I have two examples for you @Weekend.
I have one scene with one 3,7mil poly mesh with 3 levels of multires on top of it. After opening, the scene takes around 36GB of RAM no matter if the modifier is enabled in the viewport or not. After enabling viewport preview for the second level of multires the counter shows around 19,1 mil faces. Unfortunately I cant tell how much the third level takes, because Blender can’t handle it, and crashes. If I want to render this scene I need at least 128GB of RAM and another 128GB of swap memory. During rendering preprocessing the RAM usage spikes substantially above RAM limit. This scene takes 3,8 GB of disc space with compression, without it’s more like 7-8GB.
Second example. Scene that takes only 30MB of disc space, but after opening - 14GB of RAM. In this scene I have two big meshes. First one is a plane with heavily modified Ocean modifier - this mesh have around 7mil polys with the modifier on. Second mesh is only 1mil but with 4 levels of subdivision on top (subdiv is used by displacement shader). Together this scene can take pretty much the same amounts of RAM during rendering as the previous one.
Those are only an examples that even if you have relatively low poly mesh you can hit memory limit pretty fast. I’d second the advice about maxing out your RAM.