This is likely a limitation from Pinta 1.7 still using GTK2. You can try one of the GTK3 development builds from https://github.com/PintaProject/Pinta/actions?query=branch%3Agtk3-v2 (requires a github login to download the builds), since GTK3 has many scaling-related improvements.

Not sure why (possibly influenced by desktop environment settings?) but my buttons definitely look larger than that with a 2560x1440 resolution (which I'm guessing is smaller than a "2k" screen, I never know exactly how to interpret those one dimensional screen resolutions like "4k"). I'm using Mint 20 XFCE which is based off of 20.04 Ubuntu. Not sure if this is expected - not a GTK expert here. (Pinta 1.7)

I'd probably guess it's some combination of either the theme or high-DPI settings? You could compare against the development build from https://github.com/PintaProject/Pinta/releases/tag/dev-preview-builds and see if it's an issue solved with the GTK3 build of Pinta (v1.7 is GTK2)