If the image in question is one that has been synced in a Collection from Classic to the Cloud/Mobile then it will be a lower quality Smart Preview and not the Original.
If you have the Photography Plan then you can also import the Original into Lightroom Cloud Desktop, (and maybe Lightroom Web) and this will overwrite the Smart Preview in the Cloud, allowing full res export from Mobile.
Classic can only sync Smart Previews to the Cloud.
Thank you for answer. Last night I try to upload my picture into the Lightroom cloud desktop, and goto Lightroom ipad, wait until sync finish, download original file to my ipad, and export it(by ipad), but the picture size is still smaller. Lightroom desktop cloud is 6.78 MB and Lightroom IPAD is about 3.6 MB. and Photo shop cc is 10.4 MB