Using Sierra 10.13.6
I suddenly noticed today that my safari browser has giant tabs
I am pretty sure that earlier I had fixed sized tabs and not the ones which run across my entire screen.
Prevent Giant Tabs in Safari
safari
Related Solutions
If you have Alfred 2, there is a workflow for selecting Safari tabs by typing. It includes tabs from all windows, but it doesn't group them by window.
I just had the same happen to me, and likewise I prefer to keep my work and personal bookmarks separate. I was searching through menus and preferences when I noticed the small pop-up message at the bottom of the Favorites page display that asked something like "Favorites showing bookmarks and recently visited from Google Chrome. [Keep] or [Do Not Keep]" so I clicked on Do Not Keep and it all went away.
I don't like not being able to undo something, so I was looking for where in the settings or preferences this could be toggled, but alas no luck so I guess it's permanent.
Best Answer
Fixed-width tabs were used up until Safari 4. Starting with either Safari 5 or 6 (I'm not sure which because I can't find a screenshot of Safari 5's tab bar), the style switched to full-width. Safari 6 was released alongside macOS Mountain Lion in 2012.
So, unless you just upgraded from a version of macOS earlier than Mountain Lion, your tabs have always been full-width.
Unfortunately, I don't know of a way to change the style back to fixed-width if that's what you prefer.