I just did a quick path measurement on Google Earth to the coordinates and with combining of what we know about the path before it disappeared, it would have had to fly 5000 miles. Being that the extended range version can do more than that, it was only on a planned flight of about 3000 miles. They typically will not overload it with fuel. Not almost twice of what it needs to get the next drink because extra weight causes higher fuel burn.
Looks like they just enlarged the circle to about 3500 miles and said "there she is."
Wish I could get some of that "expanding" fuel for my truck.
Malaysian Airlines said that it had fuel for 8 hours — 6 hours to Beijing plus 2 just in case.
At 500 mph that means 4,000 miles.
After flying for 2 hours it was just south of the Andaman Islands and a a little north of the Straits of Malaca.
3000 miles due south from that point puts them right where they are looking on the Arc of the Last Ping.