365 ribbon became too big after a restart

Can you give us more information about what you mean by "too big?" Perhaps a screen shot.

There are numerous things that can be done to adjust the Ribbon. See https://answers.microsoft.com/en-us/msoffice/forum/all/minimizing-ribbon-autohiding-ribbon-toggling/5706d1c8-8e59-4514-bc0c-a091990c667f

To reduce the Office 365 ribbon size while keeping Windows display scaling at 125%, use the Office multi‑display/DPI options and Windows scaling workarounds:

1. In any Office app (Word, Excel, etc.):
   • Go to File → Options → General.
   • In User Interface options, under When using multiple displays, switch between:
     • Optimize for best appearance and
     • Optimize for compatibility.
   • Click OK, then restart the Office app and check the ribbon size on the affected monitor.
2. Make sure Windows and Office are fully updated, as recent builds include fixes for high‑DPI and scaling issues:
   • Install the latest Windows 10/11 updates.
   • Install the latest Microsoft 365 updates.
3. If the ribbon remains oversized after a restart, try these Windows display adjustments (they affect all apps, but often normalize Office scaling):
   • Right‑click Start → Settings → System → Display.
   • Under Scale and layout, confirm both monitors use a similar scale (for example, both at 125%). Large differences between monitors can cause Office UI elements (like the ribbon) to appear too large or too small when moved between screens.
   • Sign out of Windows and sign back in to apply scaling changes cleanly.
4. If the issue started after docking/undocking or changing monitors, sign out of Windows and sign back in. This forces Windows to recalculate DPI for each display and often corrects Office ribbon scaling.

Office does not support setting its own UI (including the ribbon) to 100% independently of Windows DPI. The practical approach is to tune Windows scaling and the When using multiple displays option in Office until the ribbon size is acceptable on the 125% desktop.

References:

• Office apps appear the wrong size or blurry on external monitors
• Known issues with Office and Windows 10
• Windows scaling issues for high-DPI devices (Surface Pro 3, Surface Pro 4, or Surface Book)
• Handle high DPI and DPI scaling in your Office solution