Narratively it makes sense for HK amp to be bigger than DR amp as it is a special power imu can give to only 13 people.
You think gorosei are 3x characters like roger , WB , Garp? Because HK harald and DR xebec are 2x Roger , WB , Garp.
I don't see power scaling in that way .
Don't think DR rocks was 2x times stronger than garp and roger even if it was a 2 on 1 .
Plus the Elders have MZ that awaken who know what hax shit they could have .
Right now what hyping up loki is DF powers and you never know how crazy or nerf they can get .
Also narratively we know why the HK more important story wise but that very differnt from the buff power aspect .
They can effect Imu if hurt , he can control them from any way , there is no way to undo the contract ( at least for now ).
Hell if i want drive agenda if the roger pirates could not even kill a HK then the elders should be way out there league .
Since they have a higher mark and CoC lol.