As SSD is getting cheaper, I find myself keep thinking that I should buy a bigger one.

But this isn't really a decision based on actual need - and I can tell my mind is playing tricks to convince me that "you need to get it because what if you want to install a lot of large games".

I'm thinking may be a reasonable criteria instead is to see my current utilization, and buy one only if the utilization exceeds a percentage - say ">80% disk util=> buy a new one" as a rule?

What is your criteria to get a bigger disk?