Not sure if this also applies to iPhone 6 Plus.
A friend of mine came from an iPhone 5s, and changed to an iPhone 7 plus (256GB)
She was unable to get her backup onto the new iPhone 7 Plus due to the 256GB memory of the iPhone not being compatible with the older one.
As i said, not sure how this translates to iPhone 6 Plus going to iPhone X.
She had to make a full backup onto iCloud and then use the "restore from iCloud" option.