Alduin should be bigger, and a HELL of a lot more scary. Although this could be helped by him being bigger. He is after all, responsible for creating Mehrunes Dagon as we know it....and eats him.
Mehrunes Dagon was once the Leaper Demon King, a kindly leaper demon and a low spirit who was one of the last spirits to be eaten by http://www.uesp.net/wiki/Lore:Alduin at the end of every kalpa. Dagon plotted with thehttp://www.uesp.net/wiki/Lore:The_Greedy_Man over several kalpas in an attempt to kill Alduin. Together, they distracted Alduin at the kalpa-turning in order to hide parts of the world (http://www.uesp.net/wiki/Lore:Mundus) away from him. At the start of the http://www.uesp.net/wiki/Lore:Mehrunes_Dagon# kalpa, they would add these stolen pieces of the world, hoping to eventually make the world too big for Alduin to eat. Alduin eventually learned of their plan, and cursed Dagon so that he could no longer jump. Alduin banished him to the http://www.uesp.net/wiki/Lore:Void. Dagon was tasked with destroying all in the world which was stolen from previous kalpas, but he could not enter Mundus except during an auspicious time after a lot of hard work and ritual. Only after all fragments stolen from previous kalpas were destroyed could Dagon regain his "jumping kind of happiness" again, but the spirit knew this to be impossible, as he and the Greedy Man had stolen so much from the previous kalpas and had crammed them in at random. Dagon then begged Alduin one hundred thousand and eight times to retract the curse, but while he was begging Alduin devoured him and began a new kalpa.