I too am having this issue. I just deleted a ton of photos from Lightroom Mobile, but my iPhone 7 does not show any difference in the gigabytes being used by the app in settings. The photos have been removed, but the app is taking up 10.35 GB of space on my iphone.
What is going on here?
When I plugged in my iPhone into my laptop and then looked in iTunes under my iPhone->Apps->Lightroom I saw that there was a 5GB directory shown. I copied this directory to my laptop and inspected it's contents. One of the sub-folders had a whole bunch of pictures that I had imported into Lightroom mobile but had since removed from my mobile collections and were not in the Lightroom Mobile "Lightroom Photos".
I ended up deleting Lightroom mobile from both my iPhone and iPad and then reinstalling. This deleted the folder I had found on my iPhone from within iTunes and thus Lightroom mobile is now taking up a reasonable amount of space. This is a workaround. I hope Adobe corrects the issue within Lightroom Mobile as I don't want to have to delete and reinstall Lightroom Mobile every couple of months.
I'm suffering from same issue. I'm using now Lightroom for iOS more and more due to HDR mode added during recent versions. I typically shoot a few dozens of pictures during the days collecting some 2-10GB of pictures, which I delete once I have synchronised everything with Lightroom Desktop and imported the pictures to offline storage. Despite seeing the pictures vanish from all synced devices Lightroom Mobile continues to occupy gigabytes of storage, trying to clear cache doesn't help.
Similarly, I can also see now via iTunes the Documents -folder with all the old photos since I reinstalled the app - following exact same findings above.
I contacted Adobe support and funny enough, these symptoms were known, and reinstall the app as a workaround seemed to be completely normal standard practise to solve the issue. Ehh????
Adobe, please fix this bug as it's indeed annoying to keep reinstalling the app all the time + also iCloud backups gets grown to dozens of gigabytes unless one disables backup for the specific app.
Ah, interesting! I might have found better workaround - or then this issue was just fixed.
I reinstalled the app again seeing Documents & Data occupying 2MB or so as fresh install. I took random 35 pictures seeing the storage consumption to rise to 772MB. I then waited for the sync to complete, and this time instead of deleting the pictures from desktop Lightroom, I first cleared the iOS Lightroom cache via 'Clear Cache' option. To my surprise, this action did work now as expected, and I saw the consumption of Documents & Data to again hit 20MB or so.
So unless this was just fixed in latest point release, try to first release the storage by clearing caches BEFORE deleting any pictures. Only once this action is done -> remove / delete synced pictures or pictures from the device.