150 HP vs 225 power difference

[Donzi111]

Maybe a dumb question, but if the props are exactly the same, and the running rpm is the same, why is more HP needed? I noticed that a 150 HP used the same prop as my 225, or so they stated.

Also, been told that OB's should be installed so that cavitation plate meets bottom of boat - why are I/O's installed "much" lower?

[boats.net]

Maybe one boat is heavier than other one.
Don't know that I/O's are that much lower than an outboard motor but boat manufacture sets each up for best performance.