not pixel density. The wasted space it just a lot of added white space and banners/icons.
The icon for a notification takes 1/3 of the screen. While in the timeline, you generally have white space equal to 1/3 of the screen so you get an item at the top and 2 lines of the next item at the bottom.
It should all be fixable in software, but I'm not sure if they are going to give up their "pretty" ui