Taken from the Trivia section of wikia's Spellbreaker page:
- Though it functions like a shield, Spellbreaker counts as a Heavy Armor piece. Blocking with it raises the Heavy Armor and the Block skills.
In all my 2 and a half years of playing (heavily modded) Skyrim I never friggin realized shields count towards either the light amor or heavy armor skill. I dunno... maybe it's only Spellbreaker that counts as heavy armor, or maybe there's some technicalities which I have the least bit of intrigue in. Regardless, my current character in Skyrim never, and I mean NEVER, touches skills in alchemy, illusion and heavy armor, and I would REALLY hate the fact that this realization pertaining to shield types would render this somewhat trivial quirk invalid.
P.S. sorry for any bad english :/