according to the anime or manga Rudy has a lapalace factor that can make his magic power grow endlessly by leap of bounds. While kyotaka (Julian) doesn't.
My mana detecting eyes. I was now able to use them at will. They had their own uses other than just helping me to detect mana. I could use the extensive information from the composition of mana to their volume and use it to efficiently use my spells, without wasting a single drop of mana. After all, mana for me wasn't endless. Unlike Rudeus, who's magic reserves were increasing everyday. I had to eventually find a way to utilise my much smaller pool in a better way.
Others · Reprobate69
I can only make a readable fanfic though hahaha
ch 0 2 Chapter 2
Anime & Comics · KidSpeech
Fear not. i got you bro
One of the main reasons is that he has lapalace factor. It was said in the novel before
>Rudeus: Laplace factor (reason why his mana pool is so big, even more than the demon god, Laplace himself)
Jobless Reincarnation x COTE| A Masterpiece
Others · Reprobate69