Everything in life is a matter of degrees. There's smoothing out and then there's Shizzo level blubber.
But, if you want your muscles to get bigger you have to get
bigger and that's never going to happen unless you are on a caloric surplus. There's no other way
around it. Training by itself is not going to do it. Of course, if you are in bad shape you can play around
with body composition. Lose fat while gaining some muscle while overall body weight remains the same.
But if you are a seasoned bber looking to advance it's not going to happen if you stay at 6%. I guess if there were some way to exactly determine the precise amount of protein, carbs and fats that will allow you to optimize muscle growth without adding an ounce of fat that would be ideal. But in the real world it puts the odds more in your favor to have an overabundance of nutrients at your disposal. I mean, it's hard enough as it is for a season trainee to put on additional muscle even under ideal conditions.