iPhone 6 back up to iPhone 7 without photos/videos
If you have iCloud Photo Library enabled your photos are already in the cloud and will not add to your backup size.

But you still need enough cloud storage for all the photos &amp; videos and the backup of your other data.

For example my iPhone backup is 1.6GB but my photos in iCloud Photo Library are over 80GB in size.

If your cloud storage is too small for the iPhone 6 backup you could use use a computer with iTunes to do the transfer to your new phone. Just do a iTunes backup with the iPhone 6 and then restore the iPhone 7 from iTunes.