I'm not quite sure which setting you are specifically referring to, but I'm going to take Forgotten Realms as an example. In Forgotten Realms you have deities who grant divine magic to those who serve them, and arcane users can tap into the weave or shadow weave, that both were created by Gods. Now, if you look at the non magic classes, like fighter. How is a regular fighter supposed to easily take on power that comes from the Gods?
I think it all depends on the lore. In certain settings it would make sense that magic is more powerful, in other settings it may be completely different.
Regarding realism, or believably. I think that armour and weapons should have appropriate size, and not be too big. There are other things as well, like the environment looking authentic, but besides that I don't care much. Fantasy means magic and realism is generally relative to the setting it is in.