Acronis image much bigger than it should be?

Hi. I have a new Lenovo Thinkpad Edge E440 with 1gb hdd.

It came pre loaded with win8.1 and the following partitions:
Revovery partition 1,000mb
Efi partition 260mb
Recovery 450mb
Recovery 12.5g
The remaining partition contains win8 etc but I split it into 2 using win8 storage management so I have:
Win C: 460gb (OS & programs) and Data D: 456gb.

I wanted to image the C: partition which according to explorer is 54gb used and the rest empty.

When I run acronis however it seems to image the whole partition and more as it comes back with an image of 460gb even after selecting high compression.

I've used acronis a bit over the years (this 2014 version is new because of the change to win8) so am familiar with how it should work. I did not select to backup sector by sector or unallocated space. If I try to image all partitions except D: then the image is bigger again by around 10gb.

This might be a clue - when I split the original win partition to make a partition for my data files I couldn't free up more than 456gb even though the PC was new with not much loaded. Disk defragment didn't help.

What could be causing this?