Low-res photos when transferred from iPhone

I have an iPhone 4S running iOS 8.1.2

When I take photos using the built-in camera app, and transfer them off my phone using DropBox or PhotoSync the photos mostly end up as low-res versions. Not all of them, but about 90%.

I've spent some time trying to work out what is going on and its melting my brain.

Things I know:

1. 
   

   The problem is related somehow to the iPhone camera app as it doesn't happen with Camera+.

   
   


2. 
   

   The photos do exist on my phone at full-res. If I plug my phone into the laptop with the USB cable, a drive gets mounted and I can get to my photos, copy them off and they are there basking in their high-res glory. That rules out any dumb stuff like 'was I using the front-facing camera' or 'have you set the phone to store photos at low-res'.

   
   


3. 
   

   If I use DropBox or PhotoSync they come through as (mostly) low-res.

   
   


4. 
   

   I'm not doing anything with iCloud as I know that causes lower-res versions.

   
   

Any help would be appreciated as it is doing my head in!