Icon sizes "large" and "huge" are smaller than medium (same as small)

System: Ubuntu Cosmic 18.10, Gnome 3.30
Display: HiDPI, 15.6 inch, 3840 x 2160 pixel (282 PPI)

Reproduction:

1. Start Gimp
   -> Icons are tiny
   
2. Edit > Preferences > Interface > Icon Theme
   -> Icons are set to "Guess icon size from resolution" (but they are tiny)
   
3. Change to Custom icon size
   
4. Set size to Medium
   -> Icons grow
   
5. Set size to Large
   -> Icons shrink again to same size as Small
   
6. Set size to Huge
   -> Most Icons stay the same, some shrink even further