Lyxen
Great Old One
It doesn't really matter how many HP or how many encounters. 20% more damage*
First, it's not 20%. When you take a d8 as a basis, a change of modifier from +2 to +3 it's more like 15%. The value of 20% is sometimes used to integrate, on top of that, the bonus to hit.
This effect does not only appear over large numbers. That's nonsense.
No, it's not, please read on the subject. Yes, with large numbers, it will likely converge, but conversely, with low number of stats, it means that the noise of the random factor will be a much larger part of the result obtained, and in any case there is no predictability over individual events.
This is especially true when comparing a d20 with a measly bonus of +1. The luck of the dice, but also the actual situation and tactics applied matter much more than a +1 in terms of damage dealt. Not to mention the fact that there is always the overkill factor (death at 0 hp which comes to complicate matters even more).