Why doesn't uninstalling an app free all the space it had used?
It could be a cache issue. You could try downloading  Astro File Manager and open it. Then go to File Manager > Pref > Cache from there click on Other Internal Caches. beyond that you will probably need to do a factory restore to retrieve all your memory. 

Resources:


Uninstall app doesn't change space used
  
Support Samsung Galaxy S2 Internal Memory
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