Noise is ephemeral and therefore harder to prove.

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