Kaitlin went panel-by-panel in these books analyzing various properties of their page layouts. She coded over 9,000 panels across 40 comics. Some of these features are captured in this figure:
She found that certain features have decreased over time (horizontal staggering, etc.), while others have increased over time (whole rows, etc.). Overall, her conclusion is that pages in earlier comics were fairly unsystematic in their layouts, while over time they grew to be more systematic, and at the same time more treating the page as a whole "canvas." This is complemented especially by changes towards using "widescreen" layouts in pages over the past two decades.
You can download the paper here (pdf), or at my downloadable papers page.
Also, here's Kaitlin at Comic-Con 2015 reporting on her initial analyses of this project:
Page layouts are one of the most overt features of comics’ structure. We hypothesized that American superhero comics have changed in their page layout over eight decades, and investigated this using a corpus analysis of 40 comics from 1940 through 2014. On the whole, we found that comics pages decreased in their use of grid-type layouts over time, with an increase in various non-grid features. We interpret these findings as indicating that page layouts moved away from conventional grids and towards a “decorative” treatment of the page as a whole canvas. Overall, our analysis shows the benefit of empirical methods for the study of the visual language of comics.