Hi djs71a.
Have you checked to see the details of what your iPhone SE is backing up using the steps found in this article: Manage your iCloud storage This may explain why there is a difference in size. You can see what apps are being backed up along with the data size.
"Manage your iCloud Backup
When you back up your iPhone, iPad, or iPod touch to iCloud, the most important information on your device is backed up automatically. Backups can include your documents, photos, and videos. Learn more about what iCloud backs up.
Many iOS apps automatically back up to iCloud after you install them. You can decrease the size of your iCloud Backup and free up space in iCloud when you turn off backups for apps that you're not using or delete old iCloud backups.
Choose which apps to back up on your iPhone, iPad, or iPod touch
- Go to Settings > [your name] > iCloud.
- Tap Manage Storage > Backups.
- Tap the name of the device that you're using.
- Under Choose Data to Back Up, turn off any apps that you don't want to back up.
- Choose Turn Off & Delete.
When you confirm that you want to Turn Off & Delete an app, it turns off iCloud Backup for that app and removes all of its information from iCloud.
Some apps are always backed up and you can't turn them off".
You may need to contact Apple Support to look further into this.
Cheers.