Throwing knives are weapons only used in a pinch and not particularly effective, throwing axes are in no way comparable. It's only small in the sense that it is not suitable as a war-axe, it is specifically designed for throwing. Your character is also literally launching himself forward and jumping into the air, adding his own momentum to the momentum of the axe. I don't know how strong your character's armour is, but that would be sufficient to cave in iron plate, which is all I said. I don't know why people assume I'm making OOC comments in IC all the time, how the fuck is Sigurd supposed to know how tough his armour is? He can only work on prior experience. 

Now, I can't say for certain how good a throwing axe would be, but what I do know is that ones exactly like Sigurds were used by actual dark-age armies, particularly the Franks, which puts them significantly above throwing knives in terms of combat effectiveness. They were known to shatter shields when thrown.