your best bet would probably be to upload the high res you need in the cloud to the relevant collections through Adobe Photoshop Lightroom this should overwrite the Smarts with the Originals. Maybe test it on one first (I guess also that it will also sync the high res back to the From LR Mobile Folder in LR Desktop?)
Reed's advice is correct. The original large resolution images need to be uploaded to the cloud for LR Mobile to be able to access them. LR Desktop is uploading smart previews, not the original images.
To upload the original images, import the images again using Lightroom Web (lightroom.adobe.com) or LR Mobile.
Hope this helps a bit.
Out of interest Charlie, is the syncing clever enough to realise there is already a High Res on the Desktop computer and not sync it back?
Or regardless will it just sync back a duplicate to the 'From LR Mobile' folder?