Under normal conditions. The butterfly controls the airflow, the airflow controls the fuel flow. If the butterfly is closed/throttle down, the mixture ratio remains the same, otherwise we would be running rich at low throttle and lean at full power.
Therefore if the ice builds up, as I believe it does, around the butterfly effectively closing the throttle then the same should apply, mixture stays the same, by ratio. The Venturi cross section is reduced the amount of air passing through is reduced. Although the pressure difference from atmospheric may be greater there is less volume of vacuum to fill with fuel.
Take a balloon a foot diameter at two atmospheres in a room, stick a pin in it, it goes pop. take another balloon at twenty feet dia also two atmospheres in the same room it would take out the windows, prob plus a bit! The pressure was the same.
Just my theory.