Nice theory.
Unfortunately the bridge is not moving, only the strings are.
At least on my bass.
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So your bass doesn't vibrate at all? Are you sure?
Heavier things are harder to vibrate, especially quickly, than lighter things. This is not a theory.
For this reason a heavy bridge will pass less high frequency vibration(treble) to the body, where it is lost. It will tend to reflect it back to it's origin (strings) where it will be picked up and heard.
Very subtle, more noticeably on lighter bodied basses. But very subtle.