[planb]

Might also be that newer versions of macOS need more RAM than Snow Leopard and therefore less is available for filesystem caches.

[lloeki]

This might be a factor although IIRC there has been some work after Lion (Mavericks?) where OS RAM usage went down significantly. It was really noticeable on 2GB VMs.

[saagarjha]

Mavericks introduced compressed memory, so that may be the improvement you’re talking about.