They are saying that IF the population is too high, then that could affect the size of the bucks.

Let's say the carrying capacity of your land is 20 deer. With a balanced buck to doe ratio of 1:1 you would have 10 bucks and 10 does. The odds of some of those bucks reaching maturity is pretty good. That is typically the goal QDM, to allow bucks to reach maturity and to stay within the carrying capacity of the land.

If you don't shoot does, let's say your buck to do ratio is 4:1. That leaves you with 4 bucks and 16 does. The odds of one of those bucks reaching maturity is much less than the previos scenario.

Of course if you don't control the population at all your deer are going to over populate and the result is going to be malnourished undersized deer, both bucks and does.

With a carrying capacity of 20 deer, I would rather have 10 bucks and 10 does than 4 bucks and 16 does. The way that happens is by shooting some does.