Just doesn’t do it.  I have the 3” mattress pads that are no longer available on Wayfair.  I didn’t think a half-inch difference would matter that much, but it’s not the measurement of the casing that matters, it’s the amount of filling, and the 2 1/2” pad has much less.  When I initially put it on the mattress it looked pretty much the same, but once you lay on it, it just goes flat.