You have three choices:
Comfortable - a good amount of space between emails
Cozy - in the middle
Compact - emails are tight together so you can see more on the page
You can change it with one easy step.
Click on the little gear icon on the upper right hand corner of your inbox and you will see all three options. Click on the one you want. I put all three options below so you can see the difference. The default is comfortable, so you are probably already set to that one.
You can pick whatever works best for your vision and your productivity.