Higher in the 20.
F=ma
F is force
m is mass
a is acceleration
To accelerate 1 ounce of mass to a velocity of 1150 requires a force which will be the same independent of gauge. Pressure is expressed as pounds per square inch. Force will be pressure (pounds per square inch) multiplied by cross sectional area of the bore (square inches). Since the cross sectional area of the 20 gauge bore is smaller, the pressure will have to be greater to get the same force.
The above considerations are the reason for SAAMI specifications for chamber pressure having a higher maximum allowed pressure for smaller gauges.
