Mystery Hard drive

1 TB hard drive had approximately 750 GB in use.
Made an image of the 1 TB hard drive.
Installed the image on a new 2 TB hard drive and now it shows only 488 GB in use.
No, the new hard drive is not showing any compression being used.
All programs, data, pictures and music all seem to be on the drive.
Nothing is missing that we know of.
Any Suggestions or comments?

Different sector size!!

Isn't the image that is made compressed? What software did you use to make the image?

I should explain why I was making a image in place of just copying disk to disk. The person I was doing this for wanted 2 computer identical. I made the image and let him take his original back and I then restored the image to the new computer and updated windows with a new product key. Check that all the programs would work, installed a wireless internet system for him and check that all was working as it should.

Thanks: Samuria
Different Sector Size did not cross my mind, Something to check.

Thanks: Plankton
Isn't the image that is made compressed? What software did you use to make the image? Yes, but it expands when restored to the new drive. I use a program called disk_image to make the image and restore.

Thanks: CountMike
Which program did you use for backup and did you back up all partitions ? Macrium Reflect can make images 25% smaller than source, some goes to compression and some to empty space optimization. The original Hard Drive only had 2 partitions (System Reserve & Windows [C:\]). As I mention above I used disk_image. I have used this program (Updated) for many years and have had no problems.

Thanks: Word Man
Please clarify - does "installed the image on" = "restored the image to" ? Make an image from original Disk to a external hard disc and then restore the image to a new Hard Drive