I am trying to decrease the size of my aperature library on my MB Pro HD. I used file\ relocate master to move files to my External HD(EHD) and this worked. However, the size of my MBP library has not decreased.

Relocate master (or original in newer versions) does move and not copy the master file. Jut to make sure I just ram a test and moved some images out of the library to an external drive and they were definitely gone from the Aperture library.

If you look at the images in the library that you moved do they have the referenced file badge? If you run a filter on your library looking for referenced files do the images you relocated show up? If you select one of the images and do a Show in Finder what does it show? Finally if you select the images and do Locate Referenced files do they appear listed as being on the volume you put them on?

You could go into the library and look for the masters you moved to see if they are still there but if all the above show you they were relocated then I don;t think you will find them.

How are you checking the library size? Have you quit Aperture and restarted it (better would be to log out and log back in) and then checked the size? It is possible that some of that data is cached and won't get updated till you restart.

Finally you could do a library repair in case something got 'stuck'.