You can't configure Windchill to compress your CAD Files, but you could configure Compression on the filesystem if it is NTFS - I would not recommend this, though as it is likely to slow down your Windchill System.
You can configure Vaulting rules to allow you to put your least used data on a different disk location - perhaps via a mapped network drive, but again this is not something that I would particularly recommend.
You can spend some more money with PTC and buy PTC Archive software which allows you to move old data from your Windchill System (the file data not the meta-data in the db relating to the CAD files) which will reduce the size of your vault - but be very careful about supported OS's and versions of Windchill as we had to wait until Windchill 10.x before we could actually install it in a PTC supported manner.
Really I think that 40GB/mnth is not a huge growth rate - our vault grows at about 200GB/mnth and that this is just the cost of doing business with Windchill and Enterprise software. Your IT dept should allow for this growth rate until you hit a certain size (6-10TB?) where you will need Windchill Archive to reduce your vault size going forwards.
Rgds
Gary