The one that I try to post repeatedly is cropped. It's smaller than the one I could post just now. (MB, MP)
I also tried different sizes in the app, but it doesn't work. Last week, I wanted to post a photo that was more cropped than this one and got the same result.
Did you crop it in the Photos app?
When you crop in the Photos app it does not actually crop. It sets a viewing mask. You can tell this because when you go back to the photo, after saving it, and edit again, and choose crop, you can actually change the cropping back to the full photo. Or just choose Revert at the top left.
I've had inconstant results when sharing or opening these cropped photos in other apps. Sometimes only the cropped portion gets copied/shared. Sometimes you end up with the full photo again.
That's only when you crop in the Camera Roll. Editing photos in imported albums or Photo Stream is different, because the edited version is saved as a new image to the Camera Roll.
The solution that works for me is to do the cropping in another photo editing app, even Skitch will work, and save that back to the camera roll.
Of course, I could be completely wrong about the cause, but it's something to test.
Good luck.
More thoughts.
I've done a little more thinking on this, and I think I know what triggers the full photo vs cropped photo thing. If you use Open In from the Photos app, or email, or other direct exporting/sharing from the Photos app method, then Photos intelligently sends only the cropped information.
If you access the photo library from within the app, then it's up to the app to determine if a photo has been cropped and only grab the altered image data. Some apps do this well, and other's just grab the image minus any modifying (cropping, filters, etc) data that may be associated with the photo.
I'll have to do some more testing to confirm this, but I"m too lazy to do it right now. Almost bed time.