I'm finding that Photoshop CS6 and older use resource fork to store thumbnail previews, but with API changes occurring since OS X 10.4. such requests for the resource fork actually end up as an extended attribute. This appears with xattr -l as com.apple.ResourceFork. In two example files they are about 50KB each.
Photoshop CC doesn't create thumbnails using resource forks anymore, thus this extended attribute isn't created. Plus the OS X Finder creates thumbnail previews directly from supported file types, even if a thumbnail preview resource isn't available. And last, the Resource Manager is deprecated as of OS X 10.8.x, so I'd expect even com.apple.ResourceFork to eventually go away (i.e. any application wanting to write a resource to the resource fork will fail).
The way I'd characterize data in com.apple.ResourceFork is that it's disposable data. That is, any application developer who cared about data, wouldn't store it in com.apple.ResourceFork. And this isn't a recent sentiment.
Are you seeing significant extended attribute data other than com.apple.ResourceFork in these files?