The mod allows me to change the belt stacks but that's it. Changing the weight amount doesn't seem to stick or do anything.
Can you describe the problem further, maybe with screenshots? Because for me changing the mass the rocket can carry (the option called "Kilograms") works just fine. 1000 kg equals the standard 1 t of vanilla. if i change it to lets say 100000 kg my rockets show that they can carry up to 100 t of payload.
Or do you mean that the launchable beltstacks don't scale in accordance with the change of payload capacity? If so, I think i did that on purpose, because if I didn't the belts would a) either be stuck at 1 stack (notably the case for yellow and red belts), or b) be ridiculously cheap to launch (the case for blue and green belts), which i felt threw off the balance. To be fair, there could probably have been a better solution to this but i didn't think of that when i first created this mod.
Then again, if you want more belts to be launchable, you can just adjust the number in the mod settings to your liking.
Also note that some items have a rocket capacity limit that is "hard coded" to be one stack so to say, for example Cliff explosives, they can only ever be sent up as 1 stack. The exact way, how item weights are calculated in factorio are documented here. It seems the recipes for those cases are the ones that don't support productivity and thus their weight = (rocket_lift_weight / stack_size) meaning that even if you change the rocket capacity to 100 t you can still only launch 20 cliff explosivees at once because now their weight is also increased 100x
