got
in theory..if more fuel = more heavy = pickup slow/no power
but in proton case..if your fuel is less than half tank, u will start to feel loss of power/no pickup...u need to rev higher to get the same pickup(eventhough less fuel is lighter)
but when u fill the tank until full...in theory pickup should suck but in proton case, pickup is way better, more power...when u need power, power is there
and its not placebo...its very noticeable
many of my friend who drive proton also say the same...can't explain why but i think this is the answer

and when the tank is lesser than half tank..somehow FC is much higher since no pickup and need to rev higher for pickup