Which sphere occupies more of your screen?  (they are the same)
The effect is more compelling with larger, and more realistic, stimuli. 
Click on the image below to see a larger version.
To see a Java demo of the measurement of the perceptual effect using
a method-of-adjustment technique, visit Huseyin Boyaci's web page.
A small (1.5MB) version is available here.
A large (7MB) version is available here.

Image rendered by Huseyin Boyaci