It is just the collections that you create within Lightroom catalog and turn on the sync that appear on your Lightroom Mobile and not all the images within your Catalog.
If you will notice under the collections panel within you catalog, you will notice a double sided arrow on the left of collection names and that arrow indicated that those set of collections are synced and they appear on your Lightroom Mobile.
Once you sync the images, they get uploaded on Lightroom web (lightroom.adobe.com) and then on your Lightroom Mobile Application but it is just the smart previews for those images which get synchronized and not the actual images themselves.
you can refer the FAQ article here :
I know about the collecton but there is a new feature in the mobile version that this show way more than just the collections I have selected in the Mobile version. Look at the very first line in the attached figure. Those not just the collection photos That I want to sync. I have selected
yes, that Lightroom Photos folder is a new feature and it includes the auto import of your images in camera roll in your Iphone/Ipad and the synchronized collections as well.
Ugh, that isn't quite the answer I was looking for. It doesn't cover the fact that there are around 13,000 photos listed in that location. So, I did some more digging around. I see on my lightroom.adobe.com account that my synced catalog shows more than 13,000 photos. How did all those photos get there? Do they represent every photo that I have ever synced to my mobile app and even though I'm no longer have them selected for syncing they still exist on my online adobe account? I need some one to confirm that statement. Or do those photos represent something else? How do I remove photos off the online server? I just need to know the process which those 13,000 photos got onto the server so I know what to do when I'm working on sensitive work photos that can not be uploaded to an outside server. Does that make sense?
Thanks for the help.
Ok, did even more digging. I think I have convinced myself the 13,000 photos represents EVERY photo that I have ever synced to my ipad even if you have stopped syncing them to your device. The one exception would be if you have deleted the photo from your catalog. Is this correct?
Yes, that is correct. All Photos is essentially a running tally of the images that you have ever synced. It represents those that are currently in a collection that is being synced and those that are not currently in a synced collection but had been uploaded in the past. This tally should clear if you go to Lightroom Preferences > Lightroom Mobile and click the Delete All Data.