wax and wane

wax and wane {mainly literary}
(to increase then decrease in strength, importance, etc. over a period of time) to increase and then decrease, as the phases of the moon
— то увеличиваться, то уменьшаться; то нарастать, то убывать; то усиливаться, то слабеть (напр., о влиянии)

To wax is to grow larger or increase, whereas wane means to grow smaller or decrease
(to increase and then decrease, as the phases of the moon)