Not enough room

Can someone explain to me why if I'm imaging a drive with 25gigs to a 19 gig hard drive that I'm compressing so that it becomes more like 15 gigs, Whyam I
getting not enough room for the image. I tried this on 2 different computers?

 

I'm having the exact same problem. I've created a partition of my main drive (through Acronis) that's 42gb. In the screen prior to creating the backup, I'm told that the maximum compression option will need 30gb of space. I wait 3 hours, and with 20 minutes left to go, it stops the process because I'm out of space. I'm a bit frustrated...

 

Most likely due to the fact that the compressed image size calculated (guessed?!!) by TI is only an approximation. The actual compression/file size achieved depends on the type of data being imaged e.g. jpeg, mpeg, mp3 and avi files = very little potential for further compression. TI carries out "in-use" sector based imaging and I doubt whether it actually analyses the data in these sectors.

Regards