The biggest reason the icons were downsized was because each row in the window had extra spacing removed, leaving less space for the icons.
If you look at this attached image, you can see the cure icons have no spacing between them and the next row, but only 1 pixel of padding on the horizontal plane.
The group window was kept the same for consistency, and many have actually requested "smaller smaller smaller!"
The only window that has default size (or I believe to be default) cure icons are the player window, which makes it more apparent when you yourself are debuffed.