SCSS Optimization

example.scss

@mixin content-middle {
  width:1100px;
  margin:0 auto;
}

.top {
  height:123px;
  background-color:#fff;
  .content {
    @include content-middle;
  }
}

.middle {
  @include content-middle;
}

renders_as.css

/* How it currently renders */
.top .content {
  width: 1100px;
  margin: 0 auto;
}

.middle {
  width: 1100px;
  margin: 0 auto;
}

what_id_like.css

/* What i'd like */
.top .content, .middle {
  width: 1100px;
  margin: 0 auto;
}

Hey mate,
You need to try to convert that mixin into class itself or a placeholder and then extend it to use.
You will get that rendered as "what_id_like.css"

Mixins are mainly use to override the properties as and when require.
I mean to say as a functions.

Hope this helps.