Prevent Giant Tabs in Safari
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.