Ha ha! lol, I'm using an MX400 and it's currently showing it's age. I loved the grip on that thing! . So to answer your question : I went to the store to look at the G500 and I realised it was bigger. Bigger since your thumb was supported by the big piece coming out of the mouse. I must say that I was skeptical at first but I came to like the feel of it. In general, I would say that the G500 has a different feel to it but it's a better one. There are a lot of features on it that can rival with 100$ mice. All in all, you can't go wrong with this one, if you liked the mx400 like I did, you'll love the G500.
In the end, it all comes to have you grip the mouse. If you liked the MX400 you are a palm gripper which means your palm touhes the butt of the mouse when you move it. You should then buy Logitech products. If you move the mouse with your fingers, you're a claw gripper and you'll love Razer products.
Also, find yourself a nice mousepad to go with it... it ameliorates the overall enjoyment.
If you have other questions don't hesitate