Cannot explain size of screenshot PNG file

I am running ubuntu 64-bit on a virtual machine and took a screenshot of a part of my desktop using the shortcut Shift+Print. The .png file consists of 1500x1078 pixels and has a total size of 30,1 kB.

To get a better understanding, I asked myself how to arrive at 30,1 kB?

Adopting a sloppy calculation, (1500*1078*bit_depth/(8*1024)), gives approximately 197 kB, assuming a bit_depth of 1, which is probably much higher such that the size is even larger and quite far away from 30,1 kB.

I have to admit that I am a newbie in this topic and there is an easy explanation for my question. Any input is appreciated!

The data in PNG files is compressed, i.e. redundant information is removed. Screenshots in particular tend to contain runs of identical pixels and other features which can be compressed, so their file sizes are usually much smaller than a one-to-one pixel storage would require.