Export Issues since upgrading High Sierra

I am having issues exporting edited photos since the upgrade to High Sierra. It exports uncropped photos ok, but anything cropped is an issue.

For example, a JPEG photo which is 4000x5000 on photos (8.6mb) , exported JPEG Maximum resolution and full size, sRGB, exports to a 1028x819 photo at 713 kb - clearly not enough to maintain high resolution.

I can't see I am doing anything differently - is anyone else having issues or can think of a fix?

Thanks