Bigger engines have more material to hold heat, and are burning more fuel per cycle. They even use a different glow plug. OS recommends the #8 up to a .61 engine, and the newly named #10 for over .61. Of course it's just a recommendation and you can use whatever makes your engine work the best. Long story short, it's ok that the 75 is a little hotter. As long as your in the proper RPM range and not running lean you are good to go.