Spells are weaker because you wear some metal instead of cloth?
How exactly?
Mana depletes more rapidly, like stamina, when you wear heavy armor?
I think that somehow is related to restrictions of hand gestures, overall feel of body and ability to concentrate
But there should be ability to reduce this effect (under Heavy armor skill)
One of other explanations is that most metals negates magic effect
But this theory brings 3 problems
1)some light armors are made from metal too
2)why armor made from anti-magical metal don't provide additional protection against magic?
3)what to do with heavy dragonbone armor?
So explanation is complicated
But Heavy armor should reduce effectiveness of spell casting (-50% at least for wearing full heavy armor set- those -15% in TES4 were just ridiculous)