Canonically they're bigger. There's a whole bit about it in Da Uvver Book (one of the books from Gorkamorka - it established much of the modern Ork lore).
Ture true, from snotlings to warbosses.
I just thought it was funnier to highlight a different hierarchy system since the imperium of man was measured by size.
Emperor of Mankind doesn't make sense. Unless there was a program to install the emperor from the beginning, implying a continuity of power from a previous era, there is no way the emperor is actually that big. He's a just a regular human in chonky power armor. Big brain, small body.