The volumetric fluid flow through a restrictor is proportional to the throat area and the square root of the pressure drop across it.
The mass flow rate is the volumetric flow rate multiplied by the density
We need to mainatin the correct fuel:air mass ratio.
We could consider our carburettor to consist of two restrictors. One is the throttle valve/venturi and the other is fuel jet.
Let's suppose that we have no ram effect and the float chamber and air inlet are at the same ambient pressure.
The pressure drop across the throttle valve/venturi is ambient pressure minus venturi throat pressure.
The pressure drop across the fuel jet is also ambient minus venturi throat pressure.
If we ignore the effecst of temperature, Air density varies with altitude but fuel density is constant.
The cross section of the throttle/venturi and that of the fuel jet are such that when we have sea level air density, we get the correct fuel:air mass flow ratio at sea level.
If we now climb to altitude while maintaining the same pressure drop, the air density will decrease while the fuel density remains constant. This will cause the air mass flow rate to decrease while the fuel mass flow rate remains constant. This in turn will enrich the mixture.
So to restore the mass flow balance we need to reduce the size of the fuel passage.