I only have 2 large hemparoos, so I layer 2 small ones on top of a large for nighttime, or one small one on top of a large for daytime. I did use just the smalls until I got the large ones in a swap...they worked fine. I think it might be a problem if you notice they are completely soaked after a change.....bigger inserts just means more absorbancy, but it also evens out the fit.