What's this chip (below picture) and why in some consoles it is replaced by big one ?
this chip is on top left side of the console's motherboard.
(because the file is big, i put link here)

http://pictures.xbox-scene.com/hsdemonz/36...om/IMGP1701.JPG

I am going to guess that MS found the same chip, only smaller. But they had both of the big ones and small ones so made boards that could hold either one until they ran/run out of the larger ones, and they will soon stop making boards with the solder pads for the larger one. Thats my guess, I could be wrong

It's the Ethernet chip. They're the same IC, just in different packages, the board is laid out to accept either package style. Might be cheaper to use one over the other in some cases or like Xboxplaya978 speculated they may have a heap of one tat they know is going to the smaller style and made it for both.