That would be more relevant id the choice was 15GB vs 16GB. However it's normally 8 vs 16 and that 8 is used by many processes. So, it can easily be comparing 0.5 of free memory vs 8.5 GB of free memory.

_something_ about the browser (either chrome or firefox, fresh profile, no extensions) is eating ungodly amounts of ram however, and it tends to be at its worst with JS heavy sites.

EDIT: no addons either