Hey, yes exactly amplitude is based on background size. But there is certainly a bad computation here because for small res the % of amplitude to have similar effect should be the same than high res, I'll check that, thank you.
In the future I would like to improve the effects system to make it easier to manage.
Edit: after double check, the computation is correct, it's just because with high res a small amplitude is more visible than with low res ( as it can be under 1px)