Comodo backup changed my target external Hard disk size

I am new to Comodo. I dowloaded it (version 3) installed and tried to backup a full hard disk to a bigger (double the size) external hard disk. To my surprise the bigger disk got the size of my backed up disk.

I tried to reformat it but never got back my original size. Uninstalled Comodo and reformatted my external hard disk but it did not helo.

Can anyone suggest what to do and this happend?

Please go to Control Panel → Administrative Tools → Computer Management → Disk Management.
Right click on the volume on external drive and choose “Extend Volume”.

This should fix the problem.
I assume you used clone operation which mirrors the source partition to destination partition, including size.