See  issue 802027  and issue 803977 and  issue 800329  for some context. TL;DR:

How we determine quota is mysterious. E.g.

* normal: pool is 1/3 of disk size (but keeping 10% free), origin is 1/5 of pool
* incognito: pool is 10% of system memory (but max 300MB), origin is 1/3 of pool

This leads to lots of developer "WTF???". Devtools might be a good place to surface some of these numbers and how we end up with the magical result.

(Or maybe we write an awesome page about it and link to it from the Application / Clear storage page in devtools)

Either way, we'd need to remember to update when we alter the quota calculations (e.g. if we increase the pool fraction on just CrOS) but those should be relatively infrequent.