I thought the leading number was for when very large changes are made to the core software that make it unrecognizable from a previous version. Like if you changed the render engine or the user interface, or all of the network code.
But are you proud of that large change?

If I were the author of GIMP I wouldn’t be

B’DUM’TSH

In semantic versioning the first number is for any change to a public API that is not backward compatible. It could be incredibly small, like fixing a typo, but if it changes the API your users are using in an incompatible way, you’re supposed to bump that number.