The speaker on the Series 2 was improved over the first generation Watch by my uses. But the speaker on Series 3 is much more audible and clear, which is noteable when you use audible Siri.
It’s kind of hard to tell without comparing them side-by-side but I’d guess the Series 3 is about 20% louder. The first couple of times that Siri talked back to me I almost jumped because I wasn’t expecting my watch to reply.