Hmm... I still find browsers more memory blind than anything, any machine with less than 16GB of RAM I find really struggles with JavaScript heavy sites, god damn JavaScript.
Your JS heaps are not approaching 16GB unless you have thousands of tabs loaded. You can verify this in about: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
Maybe you have some random addons(not extensions) installed?
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.
It's probably a combination of all the caching browsers do to improve observed performance and sloppy memory management in the browser software, not javascript.