I'm aware the photo quality isn't superb. The difference is a bit more noticeable on my screen. Shell icons aren't affected by this, which is why I had Explorer pinned on both screens.
Anyways, someone said on another site that the thumbnail gets loaded and enlarged when running a higher DPI. This makes sense, so I tried setting the default thumbnail size to 48x48 in the registry but had no luck. I have a theory about it loading the 100% DPI version of the icon and scaling it up, since my Firefox only has 48x48, 32x32 and 16x16 versions; the two latter ones being way too small to apply here.
Scrolling with my mousewheel on the desktop until I got a matching shortcut makes it slightly more grainy than its pinned counterpart, but still less grainy than its unpinned one. 48x48 and pinned.
48x48 and unpinned.
I'm running native res., ClearText is calibrated and sharpness has no influence. Clearing icon cache and thumbnail cache has no effect.
I noticed that on all three variations of icon sizes (small, normal and enlarged by StartIsBack++, aka W7 style) the icons got pixellated when not pinned. Changing back to 100% DPI resolves my issue.