Hi!
Ofcourse the fuel pressure will change from full tank to empty! That's why we modelers set our engines a little rich at the beginning of the flight so that the engine doesn't stop when the fuel level is near empty.
Our engines are airpumps! They suck air through the carb and blows out air combined with fuel vapor trough the exhaust.
If they don't have enough piston to cylinder fit or...if the fuel level changes too much they cannot pump air as good as they should and the result is that they (The engine) stop!

This is the reason why we set our engines a little rich at the beginning of the flight, to help the engine from fuel starvation when the fuel level (pressure inside the tank) gets lower as the tank empties.
And that's why a suitable tank size for a .40-.46 two stroke sport engine is 8-10oz. Not a 14oz tank!