System icon and "theme" size way too big after reboot

Hey there,
this is my first time installing Manjaro and I was just about to update my Nvidia drivers (using a GTX 980).
After the installation I had to reboot my system.
But after the reboot everything, except applications which use their own general size, are taking up like 60 to 70% of my screen.
I am not able to find a setting I could change in order for this to be normal again. I looked in the “System Settings” and in the “NVidia X Server Settings” (have I installed a wrong driver, as this one says “server”?)
Neither am I allowed to upload links, nor media. I would love to provide a picture of what I am talking about, but I don’t know how. Pls tell me how I should upload media if needed.

One quick addition, which could give a better picture of what I mean.

In firefox, the “tab-bar” is around 1cm thick. The “Window-tab” thing (which stores the minimize, maximize, close, title information) is roughly 7cm thick