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.
Changing Display Density - Instructions    

To adjust this limit, just follow these steps:
Click the gear icon at the top corner of your inbox. Choose from Comfortable (least dense), Cozy, and Compact (densest).If you don't see an immediate change, it’s because your browser is currently smaller than permissible with the selected density. In this case you’ll see that the setting is annotated with “on larger screens.” We also annotate the density that Gmail is currently using with “current view.”
Changing this setting doesn’t affect any other Google products you use.