Powder in 20g v 12g

With two 24g 7.5 cartridges both 2 3/4” what is the approximate quantity of gunpowder in each of a 12g and 20g case. I imagine the smaller 20g will have less but does this mean the shot (24g in each) will travel less distance with less force?

Interesting question . As I’m waiting in for British Gas I thought I’d have a look at the  Hull High Pheasant Extremes that I have in 20 and 12 gauge . Both 28gm no 6 ( U.K. 5.5). At 40 metres , they are both,  according to the spec sheet travelling at around 210 metres per second . Both of them kill average real life pheasants like for like .  I suspect it’s not a case of more or less powder , but probably a different powder possibly ? but that’s nothing more than a guess .  So on paper they do exactly the same job .   I suspect you could find clay loads that are like for like too .  I shoot practice clays sometimes  with 24gm CT 20s they are as good as 24 gm Eley First 12s.  

The 20G barrels are smaller in diameter, so the volume of content (not sure if that is the right term) is lower. The powder ignites and turns into (a lot of) gas. The expansion of the gas yields pressure, pushing the projectile out the barrel at speed. Less room means higher pressures can be reached with less gas - so even if the 20G has less powder, it could still reach the same (or higher) velocities as its 12G cousin. If the cartridges were like for like in powder content, I would imagine that the pressure would (continue to) build quicker in the smaller barrels and the 20G would have a higher velocity at muzzle than the 12G.