Yes! So glad someone else noticed this - exactly the same happened to me, and I hate the new waste of screen real estate! I hate it even more that ms thought it was ok to change a bunch of stuff without asking me whether it's what I wanted, or giving me the option to refuse.
I posted for a fix here: https://answers.microsoft.com/en-us/outlook\_com/forum/all/the-format-of-my-email-messages-has-changed-how-do/4f109737-5323-476d-ab74-76da7b6cc818
and got a reply about how to roll back to a previous version. Apologies, I don't know how to link directly to that reply post, but it's the second post from Diane Poremsky.
Update: My version wasn't listed on the latest releases, but had a lower sequence number than the 29 March update, so following Diane's instructions, I reverted to the 12 March update Version 2402 (Build 17328.20184), which fixed the problem and put everything back the way it was.