My experience as a pick player is the opposite. I like to use a thin pick and using thinner strings is has helped my tone. I like articulation and I get that with thinner strings.
For starters, thinner strings are less bright than thicker ones. You don't lose brightness with thicker strings. Downtune your A string to E and compare your two E stings. You'll find that the thicker one is brighter.
I just don't find this to be true at all. I've been swapping strings on the same bass all night and the lightest gauge is much brighter.