ORIGINAL: aerowoof
it's called head pressire and fuel draw.a large tank that os full has alot mire head pressure than a small tank.the fuel will want to gravity ffed to the engine so you atart with a needle that is closed more than normal.as the fuel depletes the head pressire decreases as does the fuel draw and the engine starts runnong lean and dies or overheats and gets damaged.
If the engine is positioned correctly, with the spraybar of the carb within 3/8" of the centerline of the center of the tank, any engine will gravity feed from full to half a tank. The pressure coming from the muffler to the fuel tank is just barely over atmospheric. Whether the tank is full, or you're running on fumes, there is the same ammount of air pressure in the tank. The head pressure comes from the weight of the fuel from the full to 1/2 tank. If your engine is set up correctly, although it is harder with an airbleed carb setup, the little bit of pressure the tank gets from the exhaust should be more than enough to assist in drawing fuel. The biggest problem I have seen with bigger tanks isn't so much the weight itself, but where the weight is. All the extra weight is forward of the center of gravity making the plane really nose heavy. You could compensate by moving the tank farther back, but
then you would run into head pressure and draw problems, but it's nothing a #3 bolt through the end of the muffler won't cure...