Hey!
Have you tried enabling "scale control" within the editor for your wave? That should give you options to adjust the scale of a wave over time. Here's a wave I made just now that I think has the effect you're after?
Adjusted the wave here to go between 0.8 and 1.0...
Also just realized that both curves end up being called "Curve X" in the editor... I'll get on that for the next update, haha. Anyway, I hope this helps!