(For example, 512,1024,2048, etc...)
https://www.wolframcloud.com/objects/sw-blog/capanels
These panels are fairly small rectangles clipped out of a larger plane, so it could be that there are filled cells elsewhere that don't show up here.
However, if you think about how the number 512 looks in binary (2#1000000000) you can see why the rule would make for a lot of blankness. Only one bit is set, and that bit corresponds to a pattern that doesn't occur in the initial conditions used by this page, which has only one cell filled in. If you ran this rule with different initial conditions, you would get a different image.
Also note that you're looking at rules which are powers of two, not just multiples of 512. (For example, rule 1532 is not entirely blank).