I'm no expert, but here's my take on it: "eating less" means you do not gain mass. Period. If you're eating in a caloric deficit - meaning you take in less than what you use - you will not gain muscle mass. However, you can 'clean bulk' - meaning, you put on muscle by eating clean calories and clean protein/fat/carbs, so you don't put on fat with it. But eating less than what you need means that you will not add mass, and probably not add any poundage to your compound movements either.