Wrong, in fact the opposite!
Because it is a percentage of your magicka rather than a flat cost to magicka PC with larger amounts of magicka will have a slight advantage but only by a little.
1000 magicka - %10 result 900 magicka left. this means this PC could "act" 10 times before they are out of magic.
10 magicka - %10 result is 9 magicka. this means this PC could "act" 10 times before they are out of magic.
Get it?
Now where it is an advantage is in how hard you hit with a spell in some cases, but this is true of fatigue in mêlée combat as well!
The more fatigue you have in reserve the more damage you do with your weapons. The Magic combat in my mod is balanced in the about the same way mêlée combat is balanced.
And on top of that you are not allowed to put in massive amounts of magicka into a CM spell unless you cast the spells properly and have the stats to support it.
As a "weak" magic user you are not normally going to have a great body with lots of endurance. With low levels of fatigue you will DESPERATELY need to steal fatigue as you throw a series of punches, it goes fast unless you only one punch "every once in a while".
If you train your body to be strong as well as practice your magic lessons, good for you, this will give you an advantage over those that cannot or do not (in combat anyway).
Combat Magic (if I do it right) will require you to have a balanced PC ( physical and mental stats) to excel in this martial arts form.
It seems interesting to say the least. Though I am still a bit shaky as to why I am absorbing their fatigue at all. I will be interested to see how this all pans out as the mod is kinda becoming more and more mana intensive (with the 1 exception of absorbing spells by punching them). Also with most of these abilities be based in power off of "maximum/current magicka" I can see this becoming a balance issue, giveing huge advantages to certain races/birthsign combos as opposed to making these effects based off a skill such as destruction or hand to hand itself.
With the current implementation it seems like high elf apprentices with multiple "fortify magicka" enhancements will be the way to go.