IMO most .45 cans I have heard on a 9mm *sound* quieter than the corresponding 9mm can. I am sure with some type of analyzer you could see but the tone/pitch is must lower and therefore sounds quieter.
We ran a test one day at work with a HK USP 9mm with a gemtech mk-9 (or whatever the GIANT can is), a prototype short 300blk can (2" diameter) opened up for 9mm, TiRant 9, TiRant 45 and Osprey 9.
The two large cans (mk9 and 2" can) sounded much quieter to the ear, sounded like a baseball bat hitting a pillow. TiRant 45 sounded similar but a little louder than those and the TiRant 9 and Osprey 9 sounded the loudest/highest pitched.
On the meter however it was exactly how one would imagine. TiRant 9/Osprey/TiRant45/Protocan/MK9. The two largest cans metered out towards 136-137 and despite the TiRant and Osprey being 10db lower you really couldn't tell unless you were the sound meter or downrange I guess.