This isn't an RPG. It's a story. A narritive story.
That said, it's notable that Magical girls have definite and distinct power levels. And that Magery exists - many magical girl enemies have classicical 'I are the wizard' magical styles rather than a magical girl's style.
And that both are in some mysterious and subtle way /Quite different/.
But this is a narritve. It's not an RPG. Statblocks should be avoided. Realism should be inserted where plausable.
Here's my attempt.
All magical girls - yes, all of them - share one common limiter.
No matter what the source of your magic, what gear you have or how many initiations - powerups - you've gone through, your magic is /still/ using your body as a conduit. It comes from you and it flows through you into your attacks.
Even the Paradoxes - the body of their other selves can be hacked to bits repeatedly...but doing that put strain on their reals. They can only produce so much energy so fast.
This is the source of the ceiling. And Initiations - transformation powerups - and equipment can only do so much. Once you've past three total or so of either one, all you're doing is changing your format to be more counter to your enemies, or your efficency, or boosting things in ways that put more stress on /you/ rather than really changeing your power.
Even before that, the very first form your power has gets the bulk of your actual strength. Each successive one after that gives you less, and after number two, they're ususally giving you an /edge/. It's a significant one - a 10% edge, maybe - but not a huge increase; and then they give you more variety.
An edge is decisive enough for most combat and small enough for making any combat uncertain. Combat comes down to tiny little things, most of the time. Judging how much ohmph to put into /this/ attack, against the enemies defense, and how open doing so will leave you if the attack fails. Not running yourself out of energy wastefully. Hitting with the right powers.
People look powerfull when they do a lot of the tiny things /right/, but in the end, the actual difference between their raw power is not that much. The exceptions are rightly feared, but usually have their own problems. (See also, Saturn, Hayate.)
Honestly, for magical girls /elemental association/ - the combination of what exactally it is that you do do and how you do it vs the enemies defenses - is usually more decisive than one's powerup state after the first.
This is why Black Gold Saw is still walking wounded after being stabbed by Emeri, when she could shrug off the pounding Magetsu gave her. Emeri's black sword is a nightmare attack - it hurts the /spirit/ more than the body.
It's why Signum hurts the black rock team more than purely physical damage, but doesn't go to that level. Her attacks are charged with mana.
It's why, in the end, the Senshi concider Signum a 'long range bombardment, please' problem. Her style is melee/blades primary and she's very near the absolute limit for a natural body on that power level.
None of the Senshi have that. They're a blaster team. Some of them have melee /secondary/ abilities, but their basic mechanics are by signum's standards /ancient relics/ with only a passing short at upgrades into the modern age.
This is..a very simplified outline of my views on things. Once you hit a certain point in maturity - absolute power starts to look very similar between Senshi, and skill and association start to become king. Some girls hit that point way earlier than others. Some powerup routs get there easier and others harder. (The Sailor Senshi route, for example appears to be positively brutal.)
But..stuff. I've had a week, this is not complete, but it's going up.