Loess smooth curve with large range

I'm having difficulty fitting a smoothed curve to a Mie scattering function with the following command:

Loess/DEST=fitYWave /DFCT/SMTH=(0.2) srcWave=defmapqscatmap, factors={defmapqscatdiams}

The attached graph shows the output (and the two input waves are also attached). It doesn't seem to matter what parameters I give to Loess, I always end up missing the fit at small diameters. The object of the exercise is to come up with some curve which follows the overall shape of the blue curve, but which increases monotonically. I'm pretty sure the problem arises from using a least squares fit over such a huge range of Y values.

I've also tried using a low pass filter to smooth the curve, but that tends to result in undershooting at small diameters. I'm sure there must be something obvious I should be doing, but I really can't find it. Anyone? defmapqScatDiams.ibw defmapqScatMap.ibw