Exporting Health app data to new iPhone

Trying to move all my health app data from an old iPhone 5 (iOS 10.0.2) to a new iPhone 7. The health app export function produces a tiny zip file of about 20 bytes despite spending some 30 mins or so while it says "preparing".

By watching the iPhone 5's available storage while the zip file is being written I can see the available 27GB dropping to 23GB (....I have a lot of data) but when the process is complete, the saved Export.zip file is empty.

is there a limit to the amount of data that can be exported or could this be some other issue?