The disk usage on your startup disk does not reflect the amount of data that needs to be backed up; disk usage on the destination should be lower than disk usage on the source after making an initial backup of your startup disk. Special filesystem devices (e.g. filesystem snapshots) and some macOS service data either cannot or should not be copied to another volume. CCC automatically excludes these items to avoid problems while booting from the backup and to avoid unnecessary disk usage. That list of exclusions is documented here: Some files and folders are automatically excluded from a backup task.

CCC doesn't copy virtual memory, Trash, nor snapshots

The largest and most notable excluded item is the /private/var/vm/sleepimage file. The sleepimage file contains the live state of your Mac's RAM, so it will be as large as the amount of RAM that you have installed. This file is potentially very large, changes constantly and it gets recreated on startup, so CCC excludes this file from every backup task.

CCC also excludes the contents of the Trash, so you may want to empty the Trash, then compare again the source and destination.

Lastly, filesystem snapshots may consume a considerable amount of space on your source volume. Select the source volume in CCC's sidebar to see snapshot-related disk usage. Snapshots retain references to files that have been deleted or modified, they are not a representation of your current data set, and cannot be copied from one volume to another.

Disk usage math is not straightforward

Disk usage is not a simple matter of adding the size of every file on a volume. Special filesystem devices (e.g. hard links) have always complicated this math, but more recently Apple has introduced more special filesystem devices that complicate this even further. The cloning feature in Apple's new APFS filesystem can lead to a scenario where it appears that you have more data on the disk than it can possibly contain, and the filesystem snapshots feature can lead to scenarios where disk usage is higher than the total size of the files on that volume. APFS also supports "sparse" files, which consume less space on disk than their file size would suggest. CCC can preserve sparse files between APFS volumes, but HFS+ does not support sparse files, so these files consume more space on an HFS+ formatted backup disk. See these sections of CCC's documentation for additional details on working with these challenges:

So how can I tell that all of my data was actually copied?

Click the Compare button in CCC's toolbar to perform a comparison of the currently-selected task's source and destination. This comparison will help visualize any actual differences between the two volumes.