For a given process design kit (PDK), the synthesis tool will have a few different types of transistors. They correspond to different trade-offs between size and power on one hand, and speed on the other. The fastest the transistor, the bigger it is and the more it leaks (lower voltage threshold means faster switch time, but more leakage).

For a given target frequency, the synthesis tool will always use the most efficient transistors it can. And the result is a mix, using the few available types. But the highest the frequency, the higher the proportion of faster and bigger transistors in the mix.

This is the bird's eye view and very simplified, but hopefully enough to get the idea ;)