It's possible that I'm making excuses for Aramaki here, but I don't think he was portraying himself as physically inferior to Kaido with line.
Aramaki seems to echo Akainu's desire for order; he seems to believe in hierarchies and the "rule of the strong" .
By his own twisted logic, he sees Kaido as a fellow strong person who earned the right to lord over Wano, and he's invading it because he thinks he can take it while Luffy is recovering and weakened.
Basically he scorns the Wano people because he thinks none of them are strong enough to reign over Wano like Kaido, so he wants to earn that for his top dogs (CDs and Akainu).
Aside from that, there's also my crackpot theory that GB and Fuji are secretly working against the WG, with GB pretending to be the "bad cop".
So per that idea, GB purposefully head out to Wano just to piss off the people who took down Kaido, so as to cause the WG more problems, and made sure to show approval of Kaido specifically to piss them off more.