The default layout for Outlook on the web consists of a chunky message list coupled with an always-on reading panel. Message previews, image thumbnails and attachment links are all displayed inline, so each message uses a significant amount of vertical screen space. With a little customisation, you can adapt the Outlook layout to use a compact design more reminiscent of traditional email clients.
Click the Settings cog in the top-right of the Outlook interface. Begin by changing the "Display density" option to "Compact". Next, set "Reading pane" to "Hide". These two changes have a dramatic impact. The message list now uses all the available space. Crucially, it also looks more like a traditional list, with many more messages able to fit onto the screen.
Compact mode also removes most of the visual extras, such as the sender avatars and image previews. These features can be selectively re-enabled by clicking the "View all Outlook settings" link at the bottom of the flyout.
Use the Settings dialog that appears to fine-tune how your inbox looks. Assuming you're aiming for maximum compactness, there's only one setting you'll need to change: "Message preview text." Scroll down to find it and then change the option to "Hide preview text." This will leave the message list showing just the subject line.
You can review the rest of the Layout options individually to customise your inbox to your taste. Be aware that re-enabling some settings, such as Inline attachment previews or the grouped Date headers, will cause messages to use more vertical space again. You might need to refresh your browser to observe changes you make after editing layout settings.