Display Density
Gmail automatically adjusts the amount of “density” of information on a page when you adjust your browser window or when you use different sized screens (when you view your Gmail account on a small computer monitor versus a large one, for example). The “Display Density” setting affects how much space you see between labels, around inbox sections, and between individual messages in your inbox.
If you prefer a denser view no matter what size your monitor, then you can set a limit on your default density. This setting reflects the lowest density--the most space--permitted for your account. If you make your browser window smaller or move to a smaller screen, Gmail will still automatically resize to a higher density so that you can navigate your inbox with ease, but it will never be less dense than you requested.