This is @extend big

This is @mixin big

Extends aren't like normal CSS. The specificity of the CSS it generates (how it decides which rule should apply, like, how !important it is) doesn't care about what order you place the @extend inside your own class.

Because of how extends don't drop their CSS rules inside your class like a @mixin, Their specificity applies to the order of the %placeholder itself. So, if you have multiple extends in your class with the same styles, the winner goes to whichever %placeholder is last in your SCSS/Sass file, not which is later in your class (like a normal CSS rule).

To overly-contrive the example, the %font--large placeholder is even inside the %font--small placeholder, but the font-size: 1rem still wins out.

Want to actually make "This is @extend big" large like the CSS suggests? Switch the order of the actual %font--large and %font--small placeholders, like this:

%font--small {
  font-size: 1rem;
  @extend %font--large;
}

%font--large {
  font-size: 2.8rem;
}

You can read more about this in my full article: Extensively Extending Sass @extend: How I Overused Extends