Why we always take "a" as a broader dimension in rectangular waveguide?

Please help me.
Tell me why we always take a as a broader dimension in rectangular waveguide. If we make a=b then what will happen.

The reason for the 2x size difference is that it keeps TE01 and TE10 modes from both occurring. Just one will occur over the 1.5:1 frequency range the guide is used for.

In the past Russia used a ratio much higher than 2:1 for the dimensions.