There's nothing to doubt; Contrast the statue of Azura you see in Oblivion with her Morrowind appearance.
That's not the point, I wasn't comparing the size of the demigod to her statue. I was comparing her to Mehrunes Dagon, in that statue they look about the same size but when you actually see them one of them is much larger than the other.
Any and all Gods in this universe can presumably assume any size and shape they wish. Even Vivec, who was a god with borrowed powers, has a giant form in his Sermons.
Mehrunes Dagon was http://www.imperial-library.info/tsobs/part07image08.jpg in his Battlespire appearance. He's about the size of the Cyclops from those old Sinbad movies or something. Gigantic to be sure but not to the epic Godzilla like proportions from Oblivion. He needed to be much, much bigger for what he was planning during that invasion.
When Dagon attacked Mournhold, he was probably in his giant form. And so were Sotha Sil and Almalexia.
I'm sure they choose to appear as larger than mortals due to the awe inspiring presence it projects. They want mortals to either worship or fear their physical presence - usually - so they appear tall and mighty. Sheogorath and Azura are notable exceptions due to the nature of the player character's interaction with them but I'm sure they could appear huge as well.
Hmm, well if that's the case then do you mean that Daedra lords have certain forms which they can assume anytime they want? if not, then is there a limit to how much they can extend their size? If not, then why didn't Dagon just turn into a much larger form and just stomp the Imperial City with one foot? or is it that his big :spotted owl: just didn't fit into the oblivion gate and he cannot change his form when he's outside his daedric realm? hey I just made a theory.
Too many questions I know