Allow me to illustrate with a simple example.

Imagine a rational person living a normal life with a proper retirement savings plan.
Here are the basics:
a) Monthly salary: RM20K (RM240K per year)
b) Income tax:  around  20%
c) EPF contribution:      11%
d) Additional personal savings:    20%

So effectively, their take home pay is about 70% of their gross salary. But since they save 20%, their actual spending is around 50% of gross income (roughly RM10K per month).

Now, after FIRE (meaning full retirement) if they maintain the same 10K monthly spending, their lifestyle hasn’t dropped at all. In fact, it even improve since work-related costs (commute, office meals, coffee, work clothes etc.) disappear.
That's what we call a "normal FIRE" lifestyle... maintaining at min the same real spending level as during working life.

Now if they decide to spend RM20K a month instead, that's FAT FIRE. Spending double what they used to while working, for the same location and standard of living.
If you have spare cash/income, you can try to calculate what you usually spend per month, and then pick a month where you allocate and spend double what you used to spend. It is very liberating really.

Edit: I just realized I didn't mention about this rational method. This method is using respective person's income and spending.
It implies that one needs to be realistic and know how to "ukur badan sendiri". Ie Fat Fire for a B40 is different level from a T01.