@mixin container {
  width: 550px;
  margin: 0 auto;
}

@mixin row {
  @include clearfix;
  margin-bottom: 5px;
}

@mixin columns($num, $margin: 1.25%, $padding: 0%) {
  $single-width: (100%) / 12;

  width: $num * $single-width - $margin - 2 * $padding;
  margin-right: $margin;
  padding: $padding;
  float: left;
}

@mixin container {
  width: 600px;
  margin: 0 auto;
}

@mixin row {
  @include clearfix;
  margin-bottom: 5px;
}

@mixin columns($num, $margin: 1.25%, $padding: 0%) {
  $single-width: (100%) / 12;

  width: $num * $single-width - $margin - 2 * $padding;
  margin-right: $margin;
  padding: $padding;
  float: left;
}

Return .container to 600px width
Return .container to 600px width