New Outlook updates enable you to customize your experience and help you stay focused and organized

I know that you ask for a way to get back to old view. Although MS said in another newer blog post they plan to release new design to users starting in May, maybe you somehow received it now (if you are in an organization, maybe your admin made you a part of Targeted release group, which gets updates sooner), but MS also not suggesting there will be a way to get back to old design. They are not going to maintain two designs. So my advice is the only thing you can do about spacing issues now. Unless MS releases some other options in the future.

Btw, to be sure we are talking about same things. You call it preview pane. But preview pane is the pane showing the letter itself (either at the bottom or on the right). It seems you are talking about the message list. And in your screenshot i can see less messages in the list with the new design. The setting i mentioned should make it closer to old design. I can't check that myself. I don't even have Outlook here. Just based on a newer blog post https://techcommunity.microsoft.com/t5/Outlook-Blog/Changes-to-Coming-Soon-in-Outlook-for-Windows/ba-p/394252