Why are demons so diverse in well ,everything when compared to the other races?

I'm still catching up to the current story after taking a break like a 2 years ago .So that maybe is what caused my confusion .But for majority of the other races the biggest differences in members is size ,with humans who look t be over ten feet tall or Drole who seems to be a one in a million example.

But then there's demons who consist of the royal family ,Meliodas,Zeldris ,and their dad who look more human's then the first Holy knight who's nearly ten feet tall,then we go to centaurs ,living suits of armor, a bunch of non sentient monsters who look what I imagined was under my bed , and Grayroad a giant clump of those previously monsters who some how combined and mutated to gain intelligence .

This also makes me wonder since meliodas can reproduce with other races what does that mean for Galand.

My best guess is that the Race of demons is more of a classification of all species that come from the demon realm and were created by the Demon King .This would make sense given there primarily called a Clan rather than a race from my memory.