Because your screen is 4K display. (your screenshot is 3840x2160).
Some of the stuff on SketchUp doesn’t really scale so well when using high-resolution screens.
Workaround is to actually set your system resolution to something smaller like 1080p. (although, I don’t like this workaround since I paid for all those pixels on my screen!!)
There were quite few threads about this.
Here is one: