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Probably you have lots of apps or the one you are trying to clear is large.Restart and check again.
Check if the same occurs in safe mode:
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008
Perform a software repair:
http://www.sonymobile.com/gb/tools/pc-companion/