The cases presented don't give much evidence for the impact of bigger images on conversion rates. Only the first case seems to be a direct comparison of images of different sizes. In the two others, the copy and design in general have changed significantly as well. In the second case, the image on the page is actually the same size, the button has been resized.

In general, though, I do completely agree with the article that the only way to find out what works is to test, test, test.