In a nutshell: Information Density.
While not exactly research, I can quote from Google themselves as to why they've made the changes: (From the Google GMail Blog).
We also thought quite a bit about the density of the information on
the screen in the new design. Gmail’s old design packs a huge amount
of information into a small space. While this is perfect for some,
many people appreciate a more airy design with more whitespace between
lines and elements on the page. This is especially true on larger
We wanted Gmail to be more attractive and easier to read by default,
so if you’re on a larger monitor you will see that the items in your
inbox are spaced farther apart than they were in the old design. We
believe that this results in a better overall experience, but it does
take some getting used to.
Our density settings reflect our design philosophy that Gmail’s new look should be more responsive, personal, and beautiful.
Jason Cornwell, User Experience Designer (Gmail)