My Lightroom catalog (.lrcat) is approx 500MB in size and is stored on a Google drive for the purpose of sharing across 2 different machines.
Am I correct in thinking that even if I make one tiny change to a picture, the entire 500mb catalog has to completely re-sync every time?
Yes, I think that is the way it works.
That is the drawback of having your catalog on Google Drive or Dropbox. The catalog is a single database file. It is counted as one file. If you make any modification whatsoever, then the entire catalog must be synchronized again. Remember that you aren't making any changes to the individual image files. All Lightroom changes are stored in the catalog.
One other quick question... if I take a photo on my iPhone and sync it via Lightroom Mobile but then DELETE the photo off the phone.. is it correct that it doesn't delete the picture from Lightroom on the desktop? (the picture remains in 'All Photographs')
Thanks for all your help.