Hi. I have had the same issues with my Fiat 500 (CarPlay on UConnect system).
Under iOS11, my screen appeared to be of a higher resolution, with 5 icons across 2 rows, and smaller text. Here are some screenshots:


After the iOS 12 update, the screen only fits 4 icons (which appear to be larger), and the text is much bigger, resulting in less rows per screen:


What is interesting is that the iOS 12 screenshots appear to be of a higher resolution (1206 x 720) than the iOS 11 screenshots (1040 x 624). I would have thought it was the other way around?
Anyhow, I prefer the look of the iOS 11 CarPlay screen. I had thought that there might have been a setting to choose "smaller text", but I have been looking in both iOS 11 and iOS 12 CarPlay settings, and on the head unit, and have not had any luck.
It would be great if we could have the choose of the apparent icon and text sizes. I guess this is the way that we are forced to see this in iOS 12.