Apple - Finder image icons don't display at size 16x16 but load fine at 20x20 pixels

You can modify the minimum thumbnail size in the info.plist (in .../DropboxQL.qlgenerator/Contents/):

<?xml version="1.0" encoding="UTF-8"?>
    ...
    <key>QLSupportsConcurrentRequests</key>
    <false/>
    <key>QLThumbnailMinimumSize</key>
    <real>17</real>
</dict>
</plist>

After editing in a smaller size in the plist and a reboot, you will get a preview for 16x16 pixel icons in the Finder again.

The DropboxQL.qlgenerator usually is located in /Applications/Dropbox.app/Contents/Resources/

---

Also check for older DropboxQL.qlgenerator in other places and either remove them or also modify the Info.plist.

The priority order from first to least seems to be:

• app embedded qlgenerators
• ~/Library/QuickLook/
• /Library/QuickLook/
• /System/Library/QuickLook/

While this answer doesn't explain the why, it at least explains the who. The culprit was:

~/Library/QuickLook/DropboxQL.qlgenerator

After removing it, the 16 pixel Finder icons started loading again.

Even though "Finder Integration" was disabled and the application wasn't running it interfered.

While this solves the problem, there is still a bounty to be had for an explanation as to why only the smallest icon size is affected and no others above.

Update: thanks to klanomath, looking at the Info.plist within the DropboxQL.qlgenerator showed a minimum of 17 pixels (in contrast, the system's qlgenerator shows 0):

Quick Look thumbnail minimum size 16

After editing the plist and changing the value to 16, the 16 pixel icons kept on loading.