This phone is half the cost of the S3 and has much less total functionality but I am convinced the Lumia is a better phone. I would like to convince you of this too as I believe the S3 and the forthcoming S4 are pulling the world in the wrong direction.
Personally I'm right beside Irvine in the size question, a point which I've discussed previously on All About Windows Phone, and in some cases specifically about the pocket rocket that is the 620:
Now that the Lumia 620 has been announced, there's a phone that is beginning to get me excited. But I want a nice high dpi screen, I want the top end processor (because at heart I am a tech geek), and I want a stupid amount of storage, Gorilla glass, more than a VGA and a 5 megapixel camera, high quality polycarbonate casing... Nope, I'm not allowed that. I can have the size I want, or I can have the features I want. I can't have both.
Having reviewed the Lumia 620 and spent some time with it, it does come really close - the slightly washed out colouring in the screen and slower processor are occasionally noticeable to me, but I think Irvine is going to really enjoy his time with the 620.
You can follow his reasoning and hopefully his blogging of the experience at his blog, Danielivine.com.