How can we ensure that contact photos appear as either full-screen or as thumbnails on the iPhone?
In my testing1, pictures set on the iPhone itself consistently appear as full screen, and pictures set in Address Book on OS X and synced via iCloud appear as thumbnails. Any photos set elsewhere, but subsequently edited on the iPhone will also appear full screen.
I haven't tested other syncing methods, but from what I've read elsewhere, including the question you linked, I think the syncing method doesn't matter — just whether the photo was set on the device, or on a Mac.
It's hard to say why exactly without any inside information, but I suspect it's one of two reasons:
Different teams worked on the syncing and iOS Contacts app (or at least the user facing portions thereof). Some flag gets set somewhere in the data structure when you set a photo in the Contacts app that doesn't get set when a photo is synced over. Just something that got overlooked.
Intentionally left out. Since you don't get the opportunity to preview the cropping and alignment on the device screen when setting it, it was decided to make pictures assigned off-device just appear as thumbnails, to avoid any bad cropping or weird aspect ratios.
In any event, if you want consistency, you should set all your contact pictures on the same device — Mac if you want thumbnails, iPhone if you want full-size.
1 I tested on iOS 6.0.1 on an iPhone 4S, and 10.7.5 on my Mac.