Hi there! I’ve written about this in german here, but as it helps more people in english, I’ll quickly translate my observation of a Bug in the Lightroom 5.3 file export function and the workaround solution I currently use:

It seems that when exporting files at a reduced size from Adobe Lightroom 5.3 file reduction does not work when the “Don’t enlarge” button is active / clicked. All files, at least here on my Mac OSX 10.7.5 computer, will have the full resolution of the original file (or the chose crop of it). So for the moment, deactivating the “Don’t enlarge” function solved this for me:

Obviously, this is not a final solution and Adobe should hopefully get rid of this bug. There’s a reason for the Don’t enlarge selector. But if you do not have to heavy croping and settings are reasonable (and you understand what you’re doing;), you should get around it this way.

Hope this helps some users out there….