There is a bug when "Don't Enlarge" checked introduced in LR 5.3

Work around, untick the box "Don't Enlarge" when resizing.

Some also provided a fix

http://www.robcole.com/Rob/ProductsAndServices/ExportantLrPlugin/

Bug is also reported on the Adobe Forum

http://forums.adobe.com/message/5940090

Yes, I have seen this yesterday too.
I hope the will correct it in the next release.