This will not divide the font-size. You need to use a variable:

    .icon-large {
        $font-size: 4/3;
        font-size: #{$font-size}em;
        margin-top: -4px;
        padding-top: 3px;
        margin-bottom: -4px;
        padding-bottom: 3px;
        vertical-align: middle;
    };

This will result in:

    .icon-large {
        font-size: 1.33333em;
        margin-top: -4px;
        padding-top: 3px;
        margin-bottom: -4px;
        padding-bottom: 3px;
        vertical-align: middle;
    }

If you don't use a variable, the result in the css will be as is:

    .icon-large {
        font-size: 4/3em;
        margin-top: -4px;
        padding-top: 3px;
        margin-bottom: -4px;
        padding-bottom: 3px;
        vertical-align: middle;
    }

http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#division-and-slash