Modern powders are progressive burners --> the more pressure, the faster the burn rate increases --> the faster pressure rises (spiraling upwards)
Light bullets don't produce the resistance needed to raise that progressive-pressure as quickly as heavy bullets do.
^^^^ that ^^^^
I'll add this is all about burn rate , barrel length , bullet weight and space behind the bullet as it travels down the bore .
In some ways it's simple physics . Lets start with the idea of the powder/gases expanding at a certain rate . Those gases are going to expand at a specific rate no mater what once ignited and continue to expand at that rate filling any space available until there is no longer any powder to burn . Think of it as filling a air tank with compressed air at a specific rate . A little one litter tank will fill much faster then a one gallon tank .
OK now place and obstruction in the way like a bullet preventing those gases to expand , what can happen ? Well if the bullet is the right diameter for the bore the pressure being generated by the expanding gases will push the obstruction ( bullet ) out of the way and down the bore at a sufficient enough speed to allow the expanding gases to fill the ever growing space in the bore behind the bullet . This is when burn rate and bore diameter become very important . As the bullet travels down the bore it's leaving more and more space behind it for the gas to fill . This is a good thing because the gas needs a space to expand into .
However at 2" down the bore a 9mm bullet will have less space behind it to fill then a 45 will do to the diameter of the bullets respectfully . The weight of the bullet is now also becoming important as well . The lighter the bullet in a given cartridge the quicker it can be pushed down the bore at a given pressure . Resulting in the space behind the bullet to increase faster as well allowing more space for the expanding gases to fill . Keeping in mind powders have a specific burn rate which is how fast the gases will expand . Meaning the further the bullet goes down the bore the more space there is for the gases to fill .
OK so now we have a powder/gas expanding at a specific rate and a bullet that is traveling down the bore at a rate fast enough to out run the expanding gases . If you now add weight to the bullet it not only doesn't move down the bore as fast because of basic physics you also have added more baring surface which resist it's ability to move freely down the bore as well do to more friction . OK with a heavier bullet you now have made it much harder to push the bullet down the bore resulting in the bullet moving much slower then a lighter one . But wait we still have those same expanding gases expanding at the same rate . However now you have an obstruction moving slower down the bore . So now the space behind the slower moving heavier bullet is expanding less resulting in less space for the expanding gas to fill , remember the gas is going to keep expanding at it's specific rate regardless of what's in front of it trying to slow it down . This is why you put less powder for a heavier bullet , because the heavier bullet can't get out of the way of the expanding gases fast enough like the lighter bullet can . This is also why if you want to push a heavy for cartridge bullet faster you use a slower for cartridge powder . The slower powder allows the bullet to get further down the bore before it hits it's peak pressure resulting in more space behind the bullet for the gases to expand into .
I hope I just made sense right there . I've been writing and going back a rewriting for so long now I'm not sure what all I've said lol .