Small proportional changes seem unlikely to drive big effects, unless there is some feedback mechanism that is keeping the level precisely balanced.

Such as in the body, dealing with tightly regulated and critical aspects of metabolism like oxygen consumption.

But 1% changes in oxygen should be happening all over the place.

Perhaps they are. You don't know the effect because the existing experiments do not vary or hold constant oxygen levels. All you see is the net average effect, without any sort of partitioning among causes.