This doesn't always give you the full size image. Often you have to visit the page, then search to find the image, then get the source.
Quick tip: sometimes the tiny image thing happens because you right-click to open the image in another tab before it has finished loading in the Google side-column it was in. It's good to give it a moment to fully load, than right-click. Tedious, I know, but helpful maybe.
i waited a full minute before right-click -> open image in new tab, and it still opens the thumbnail-sized image instead of the original. i tried the side-column as well and got the same result.
EDIT: strange, i tried the same process again on the same image and this time it worked! i guess it really is a cache-related issue
Great to hear. I collect historical art images on my computer as idea sparks for my own paint and photo projects, and noted the image loading thing in many of them (others do just load tiny no matter what though).