>>oh_sig+(OP)
As the other guy says, it's hard to prove in court which noise came from which plane, but besides that, they may have used weight as a proxy for distinguishing between military and civilian aircraft ("tiny noisy planes" tending to be military), and, also, perceived loudness[1] is a function of both decibel level[2] and pitch[3] (that is, a whistle will be at a higher decibel level than a drum we perceive as equally "loud"). It's intuitively plausible that larger planes tend to make lower-pitched sounds.
[1] amplitude
[2] amplitude * frequency
[3] frequency