When you are talking about cannon shells, as opposed to small arms bullets, increasing propellant and velocity is only half the story. Unlike small arms (pistol/rifle) bullets, which are generally just lead with a copper jacket, cannon shells have far more space available internally to make a multitude of different shell types, such as Armor Piercing Incendiary, High explosive incendiary, etc.
We should not only be asking what are the improvements in propellants in the Macross Universe, but to me what is just as if not more interesting, is improvements in the actual cannon shells.
Of course higher velocity is generally good, and increasing velocity provides the benefits of higher energy on target, longer range, flatter tragectory, shorter flight time to target etc, etc. But too much of anything can be a bad thing, and increasing the amount of propellant and velocity, you can get increased wear on the weapon and barrel(s) = shorter lifespan, not to mention increased recoil.
A downsize in calibre does not necessarily mean less lethality (although that greatly depends on what your target is!). I do find it extremely interesting that for the VF-25, the calibre of the gunpod is the largest ever seen in Macross. Was the VF-25's gunpod specifically designed with possible future encounters with the Vajra in mind I wonder?
Graham