Throttle body size has nothing to do with fuel. It is air control. Over sizing the TB will actually cause problems at idle and low end speeds. Not sure if your system uses a MAP sensor but if it does you'll be jacking with it. You'll have a lower idle MAP and the system will attempt to compensate for it by adding fuel. O2 sensors work on the amount of oxygen present in the exhaust. The ECU says,"Too much O2? Add fuel to even the burn". At about 2500 RPMs you will notice a slight performance boost as you now have sufficient flow to overcome the lowered MAP reading.

I doubt any increase over 10% (going to a 70mm from a 64mm would be max) will do any good without seriously improving the flow characteristics of your engine, i.e., port the heads, increase the ignition and cam timing, air feed, exhaust, etc.