How come I get more spam on my Gmail account?

I have a Gmail and a Yahoo! e-mail account.  I use Gmail to communicate with my friends, and Yahoo! each time I need to leave an address to subscribe on something.

Yet, I get far more spam on Gmail than on Yahoo!  Right now I have 162 items in my spam box in Gmail (last 30 days), including phishing attacks, but only 27 spams on my Yahoo! account with no phishing mails.

Perhaps the spammers got my address from one of those freaking chain mails I've asked so many times not to send to me?  This sucks.  When I ask not to send me more of that, the sender sometimes gets offended cause he/she thought he was doing me a favour (???), plus "what's your problem?  Just click on Delete and it's gone forever!".  How ignorant.