How are you sharing the photos? The full range of export settings are available when using the "Export As..." option, but the "Share to..." option is apparently limited and only Jpeg at 2048px is always sent. I talked to one of the engineers about this a couple of months ago, and the explanation I got was that the control that they have over what's possible in social or third-party sharing is limited by the Android operating system. The only workaround I can see would be to use the "Export as...." option to export using your preferred settings to the device gallery, then share from the gallery, which is certainly not ideal.
We can share full size and full quality photos from other Android apps like Snapseed, so I think the line about it being an Android limitation is false. This is a bug, which should be ironed out for a paid app.
Snapseed exports it full size and quality as standard, so maybe instead of offering us the choice to select the size and quality there needs to be a share in full quality option added. That is the only possible thing I can think of which would get in the way of sharing in full quality. Adding an extra option with no settings is easy.
Please add this as a feature request Jim- I have mentioned it in my own thread but I didn't suggest there that they consider adding a share in full quality option as an extra instead of having the current export options try to do it. There is defintily a way around it and I suspect it hasn't been addressed because most people don't use sites like Flickr to share full quality photos, so they are OK with the low quality files the app currently does, with Facebook's preferreed 2048 pixels as the longest dimension.
For a premium product which is used by professionals, we should have tools at least as good as free apps like Snapseed.