^because he was a Werewolf. And Mikael despised them.

It just felt he was talking about his physical appearance

I still dont get this whole weak arguement, problem of it is we didnt see enough of the mikaelsons as humans so we dont know of mikaels perspective of weak

We saw a flashback of it... Klaus was impaled by him with his sword and he was weak because of the spelled starling necklace.

I mean like the example of him punishing klaus for the sword fight but he let elijah off the hook

^Because unlike Klaus, Elijah stopped fooling around when their father showed up. Klaus started arguing.

I believe they both stopped at the same time

He thought Niklaus was weak, but I don't remember if he was shown why he thought that.

We didn't see much but even young, Klaus was probably weaker than his older siblings at the same age. He wasn't capable of hunting properly and Mikael stated that he growed more pathetic every day. Again, the disapointment factor can work here. Maybe Klaus wasn't that pathetic, but Mikael had such high expectations of him that he seemed weak.

Also, everytime Mikael fought Klaus ( as humans ) Klaus couldn't even hold his own. Prooving to Mikael that he was " weak ".