Thanks for the info, that's a bit surprising.
I have a filco with brown switches and a Cherry G80-3000 with blues. The Filco's spacebar sounds different than the other keys, but it's not that much louder. The Cherry's spacebar, on the other hand, has a VERY loud click even when I press it lightly, just like the OP says. So I assumed that the issue isn't caused by the spacebar per se, as my brown-switch filco spacebar is not too loud, but the switch.
I just yanked off the spacebar from my G80-3000 to see what's what. Sure enough, the green switch isn't louder than the others, you're right. It's a bit odd that the spacebar amplifies the click of blue/green switches so radically, but doesn't amplify the sound of bottoming out nearly as much. In fact, the bottom-out is louder on my Filco's enter key than on the spacebar. Maybe the spacbar doesn't bottom out as hard because of the way it's stabilized, I don't know.