Upon closer inspection, I observed that I had explicitly zoomed out on Google Sheets to 50%, which was affecting the zoom levels of the top menu bars in both Google Docs and Google Slides.

When I changed the zoom level of Google Sheets back to 100%, the zoom levels of the top menu bars in Google Docs and Google Slides were fixed as well.

Whatever zoom level you set for Google Slides will also apply to the zoom levels of the top menu bars in both Google Docs and Google Slides.

Here’s a screen recording to demonstrate the same: https://youtu.be/XFxf0iVg0ck.