I am right handed with pretty big hands, and I love this mouse. Here's why.
There are two main ways that people control mice: with the palm or with the fingertips. I am in a small minority because I use my fingertips. Most people use the palm; Rob, you clearly do because you talk about the MX510 fitting like a glove. For fingertip mousers like myself, mice like that are incredibly uncomfortable because they're difficult to use without touching the whole body of the mouse with the hand.
When I use a mouse like the G3 or the MX300/310, my palm never comes in contact with the mouse. My thumb and pinky touch the mousepad and the very bottom edge of the mouse on either side, and my three middle fingers' tips touch the three buttons with the rest of the fingers up in the air. My wrist is planted on the mousepad 95% of the time. Moving the mouse around is accomplished by pushing on it with the thumb and pinky, mainly.
With big "ergonomic" mice like the MX510, the angles of the buttons and shape of the mouse are very bad for this style of control. The G3 is the first mouse from Logitech in years that is good to use for fingertip mousers. Razer's mice are designed to be used this way too, but I don't like them for other unrelated reasons.
Logitech knows about different mouse control styles; it says on the back of the G3's box "Compact, ambidextrous design for quick wrist movements and fingertip control," accompanied by a very exaggerated picture of a hand holding the mouse similarly to how I do.
So that's why the G3 is small and why that is awesome. For me this is the best mouse that has ever existed.