Having troubles using the application icons due their size. Changing from "Small Icons" to "Large Icons" doesn't really improve anything. The MacBook 13.3-inch screen is set to 2560 × 1600 resolution.
I would tend to agree. Even the "large" version of those icons is hard to read and quite a bit smaller than the icons in Apple's apps (Safari, Pages etc...). Attached a direct comparison: Upper line: MP3TAG, Below: Pages
Making these icons a bit larger and in a slightly darker grey frame would certainly help, I think.
I've slightly increased the icon size with Beta 5. It uses the the standard sizes as recommended by the HIG, i.e., 19x19 (and 38x38 for Retina screens).
Florian I think you should look and read " Toolbars and Title Bars" paragraph at What’s New in macOS - macOS - Human Interface Guidelines - Apple Developer.
I think the problem is between OS 10.15 and new OS 11 UI.
I'll still send you email 
I've again increased the icon size with Beta 7, mainly for Big Sur. This should address the issue definitively.
I had to do a full rewrite, so please look out for any issues.
Finally on Big Sur "New Folder" symbol looks the same (size and thickness) between Finder and Mp3tag! 
Definitely better - thanks Florian.