I've noticed that at times, too, even on jazz gigs I engineered! I think that it's a combination of things. The musical arrangements of traditional jazz tend to look at the bass as THE low-end instrument, and it's often the soloes that get "up there". So, there is a desire for the higher notes being played to not over-shadow the other instruments in that range (horns, piano, guitar, etc), or the singer(s). Did I make any sense with that?
Then, there's the instrument itself. It's resonance point of the body is pretty low in terms of frequency. As the notes progress up the scale, there are some other resonance points, but , generally speaking, the
volume gets more subdued. And because "trad-jazz" is more "organic", the producers tend to accept that fact and leave it alone. Very little compression or EQ is used. I use a gentle ratio (2:1) with a slow release to just take off the peaks a bit. Anything more sounds unnatural.

There is a big reason for what you are hearing - that is just the way an upright bass sounds acoustically like when is played pizzacato. Yes, you can change that through amplification and recording techniques, but you are really changing the character of the instrument. Of course, everyone does this to some degree and some bassists have gone into it more than others. But there definitely a "purist" school that wants the "bass to be the bass." I have a lot of sympathy for this. Why make the URB sound like a bad electric bass? Either embrace the sound or pick up "the slab" as the electric bass is know to "real bassists." (Check out the URB pages at talkbass.com to read some real fanatics. I mean that mostly in a good way.)

One example of a bassist that goes a little farther that some others in "treating" the problems of the URB is Ron Carter. He takes some heat for his tone from purists, but he has great technique (which is a big part of the battle anyway). Take a listen to his recordings with Houston Person (bass/tenor sax).

I agree, that soft pizzacato is the tone of the upright bass.

Its up to the other musicians to give way, dynamic musicianship . Other instruments have to bring thier part down to a whisper to let a quiet instrument solo. The arrangement has to have the air and space too.

A lot depends on the intruments.

For example: An Italian bass made around the 1600's has a totally different sound (and size) than a French bass made 100 years or so later.

From the two specific samples I have heard, the Italian bass had louder upper register when played with finger while the French bass's upper register was louder when played with a bow. Again, this is from the two samples that a friend of mine plays. For Jazz, he uses the Italian bass (yes, he takes that very expensive intruments to gigs...!). For classical music he usually prefers the French bass.

Clearly, if one were to record either of those two intrsuments the choice of micophones and placement would likely vary accordingly.

But, even instruments made recently all differ in sounds. String choice affects the registers as well.