How much RAM you've allocated as VRAM for your APU's Radeon R7 built-in GPU?
Since that's the 1st thing that comes into my mind when i read your issue. While AMD APUs do have built-in GPU in them, APU doesn't contain any VRAM. Instead, it's taken from the RAM in your PC. By the looks of it, you've 16.1GB of RAM allocated to VRAM while the rest, 15.9GB is free for CPU/application use.
To change the allocated RAM value, look into MoBo BIOS.