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/):

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
    ...
    &lt;key&gt;QLSupportsConcurrentRequests&lt;/key&gt;
    &lt;false/&gt;
    &lt;key&gt;QLThumbnailMinimumSize&lt;/key&gt;
    &lt;real&gt;17&lt;/real&gt;
&lt;/dict&gt;
&lt;/plist&gt;


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/