Time-Shift function has "Size" setting. What does the option affect on ?

[Enjoy]

Hi guys, i have a question about line "Size" in settings menu of the Time-Shift function.

I made set up my Time-Shift highlight length for 30 seconds constant recording.
Then, i'm using Time-Shift and sometimes save highlights and size of 30 seconds videos what i'm getting in the end are ~ 10-20 mb, so, in my case i have constant consuming very little amount of space in RAM disk during Time-Shift non-stop recording and of course the same little size of videos i get when i press hotkey to save Time Shift highlight to my SSD drive.

My question is - what does the "size" line setting mean ?
There are many options from 1GB to 128GB.

For what is this, let me understand please.