There have been two major revamps recently of social media platforms and both go the same route, bigger images. Google+ and Flickr have changed their design and especially on Google+ the images can be blown up to really big sizes. But that only looks good when the image is high res and the question is do you want to upload high res images to social media?
Lets only take guests of photographic safaris, team building photographic safaris en attendees of wildlife photography courses. They all come home with beautiful images and they all want to share them with their friends and family. If they upload their images in high res, anybody who wants to use their images can download them or screen-shot them for further unauthorized use. The quality will be excellent and will be even fine for brochures and other print media. Now you could watermark the images, but you don’t want it to be too dominating on the image. That still gives people the opportunity to download your image and crop the part they want for their purposes.
What to do? I think it’s wise to keep using low-res images. You might not be able to avoid stealing entirely, but at least you can limit the use of the images. And you don’t know anyway, if Google will choose your image to be the huge one on the stream.