? Who's to say a fatter tone is not more harmonically complex? Lot's of people like playing higher on the neck. This shortens the scale, and on the fattest strings (E and B) can get into a zone where inharmonicity occurs...but some people like it and describe the sound as thicker. I generally like playing closer to the nut, especially on the E and B string.
I've no background in physics, so you can stop reading past this point, but I'd venture guessing that inharmonicity may be just one product of the shorter length of string behaving less like an ideally elastic body and more like a solid bar (SNEAK EDIT - you mention this in your subsequent post); another is an overall reduced capability to vibrate in short sections and produce higher partials in the first place. However, this is, again in my armchair guessing, mostly confined to high highs, i.e. "sizzle" or "air", and not directly correlated to fatness.
A much more profound effect is pickup location though: unless the instrument is piezo only, playing up the neck shifts the relative placement toward the center of the vibrating length, making the pickup, uh, pick up more fundamental as a result.
A couple of final considerations (this time not related to fretting choices):
- cabinet- and ear- bass roll-off means fundamentals of notes beyond the first available octave of the bass are more audible than those in the first octave, and will pop out more;
- auditory masking, which is partly asymmetrical IIRC (net of the aforementioned low-end rolloff, lower frequencies mask higher ones more easily than the opposite) also probably plays a role, which makes spectral analysis one useful source of data, but one that needs to be weighted against other, physiological aspects.
On the other hand, an upright bass (longer scale, thicker and stiffer strings, higher tension) puts out a hell of a lot more low end than an electric (shorter scale, thinner and more flexible strings, lower tension) in the same tuning.
They really are two vastly different instruments: for one thing, double bass strings, though sometimes pretty flexible in their own right (not just synthetic core strings, but also steel rope-cores), are flatwound and also pretty heavily damped with silk inlays, in order to make them less harsh-sounding with arco playing.
EDIT: the silk layer also increases the gauge with a minimal addition of mass, so, although high-tension, upright strings are actually lower-tension than their diameters alone would lead you to believe. With G-strings in particular, if you replaced one with a damping-less bass-guitar flatwound G (of appropriate length), it would probably break before reaching target pitch, due to it being fuller=all-metal=more massive=more high-tension for a given pitch.