Totally hear you! Fixed panels eat up way too much space on smaller screens. Just to check, are you using the classic desktop Outlook or the New Outlook (WebView-based version)?
Quickest workaround: if you’re still on the classic version, try switching back (File → Options → toggle “Try the new Outlook” off). I did that on my Surface just to get panel resizing back, since the New Outlook version locks that sidebar width.
Would be great if HubSpot could nudge Microsoft on this. In the meantime, I’d upvote any Ideas thread you find (or post one if it doesn’t exist yet) makes it easier for the product teams to push the case.