In my experience (an N of two, my wife and me), if you actually take the time to empty the spam folder, you get less and less.

My wife never looks into her spam folder, and gets 20 or 30 a day. I empty mine regularly (I have a few lists that occasionally wind up there, and need to return those messages to the fold). I typically get 5 or maybe 10 a day. Before I began actively emptying it, I was getting 40 or 50 a day.

I suspect after you've assertively deleted certain types of spam, it gets more fully blacklisted, and gmail no longer delivers it to your spam folder, even.

As far as I can tell, Google is opaque on the issue.

-jk