Aperture library size

Got a really problem with my Aperture library size.

I'm running A3 on a MacBook Pro and the internal HD was getting v full so chose to relocate masters to an external HDD which has been done. All masters now sit on external HDD connected via FW800. The old library was about 40GB on my internal drive and even though its now relocated it is still sitting at 25GB. I only have a little over 4,000 images in A3 so am puzzled as to why the library is still hogging so much of my valuable internal HD.

Have I missed a trick or is this about right.? I've searched various forums to try and find the answer but no joy. Anyone give me a few pointers..?? Thanks