There is a conceptual misunderstanding here. Raising the dose doesn't mean you have a higher efficacy, in fact it can just as easily have the reverse effect. Vaccines aren't beer where you consume twice as much and get twice as drunk.
Granted. But there is a minimum dose necessary to produce antibodies. If you are in a situation where you are guessing completely blindly (because no trial like yours has been done before), do you worry more about falling below the minimum required dose or above the maximum dose past which you get diminishing returns or even lower efficacy? I think it’s reasonable to worry more about the minimum dose because you can always half the doses later. Finding out at the end of a multi-month trial that you didn’t give enough in your doses would be a big setback. Just look at what happened with Assyria Zeneca when they started mixing up dosing in their phase III. They were the most promising vaccine last summer and they haven’t even concluded their US phase III trial yet as a result.
I mean technically an alcohol could exist that makes you less drunk after twice as much but how likely is that? IMO about as likely as a vaccine that produces less immunity with higher dosage