Why is the equilibrium macrostate of a system the most probable macrostate for an extremely large number of particles?

My text book is trying to explain the cause of the 2nd law of thermodynamics through statistical mechanics using the distribution of N particles free to move between 2 connected containers. The ratio of microstates of a given macrostate to those of the most probable macrostate is plotted like this.

For large values of N there extremely large numbers of microstates, but nearly all the microstates belong to the configuration in which molecules are divided equally

What I don’t understand is how it concludes that most of the microstates correspond to the most probable macrostate as N increases from the graph of relative microstates. When I actually plot the ratio of microstates to the total microstates, the maximum of the curve decreases and goes to 0.

Doesn’t this mean that the probability of the most probable macrostate (the one with greatest entropy) is almost 0 for a large number of particles? This clearly contradicts the 2nd law. What am I doing wrong here?