Why do iI have two backups

The backup of each device is stored separately. Restoring the backup of the old phone to then new phone doesn't delete that backup from iCloud. If you no longer need the old backup you can delete it. Go to Settings > [username] > iCloud > Manage Account Storage > Backups > [old phone name] and tap Delete Backup. Be certain that everything you expect to be on the new phone is in fact there before deleting the backup of the old phone.

Deleting the backup of the old phone won't delete the backup of the new phone.

I don't know why the size is different. One possibility is that some things you used to back up are now synced instead (e.g., iCloud Photos may now have your photos and if so they would no also be included in iCloud Backup -- see: Set up and use iCloud Photos - Apple Support ).

There is no way to merge two backup files.

If 32GB of data is really missing from your new phone, that seems like it might be easy to identify what it is. Photos/videos would be my first thought given that size.

If you are convinced that your new phone didn't restore properly from the old backup, you can erase the new phone and do it again. Note that you would lose any new data that you added to the new phone.