Class names cannot be “unsemantic”. Whatever names are being used: they have meaning, they have purpose.
When Twitter Bootstrap first came out, I rewrote the compiled CSS to better reflect how I would author it by hand and to compare the file sizes. After minifying both files, the hand-crafted CSS was about 10% smaller than the pre-processor output. But when both files were also gzipped, the pre-processor output was about 5% smaller than the hand-crafted CSS.
Again: the smaller CSS file came out bigger when gzipped.