So I decided to make a Dunmer, a more traditional one - Afaik alot of Dunmer wore either Heavy/Light armor, a long blade and magic in the other hand - Destruction most likely.
So It's kind of a Spellsword/Sorcerer, yes?

Anyway, I've got one question to ask though. Seeing as Elves are less buffed up than Humans, but the Dark Elves are one of the buffiest ones, how would a Sorcerer/Spellsword wearing light or Heavy armor Dark Elf look like? (Lore wise, most importantly) on the weight slider?

I dont want to make him TOO buffy and look like an Orc of some sort, but I dont want him to look too magey either.

Anyone that has the same character care to share their opinion?

Dude. You're playing an Elder Scrolls game right now. It doesn't matter if you make him buff or skinny, it fits the lore lol. This is the lore. Just go with what looks best to you.

There is no restrictions lore wise, so you can be any combination that you wish, Many Dunmer practice their worship of the ancient ones and so conjuration could be one to look into (as their racial ability is similar to a conjured spirit). Dunmer are far from the strongest mages around, Altmer and Breton are suppose to be the two top spell casters while Bosmer are more archery and tree hugging people, so Dunmer with the exception of the Orcs are pretty much the strongest elves around and again with the exception of the orcs, the least magical; Tho the College of Winterhold was once filled with Dunmer mages on the lore side of things as Nords do not practice much magic and their is issues between the Nords and the Bretons...

I can't really speak from a lore point of view, but I would say that the weight slider for each race represents the ordinary range of weights for that race, probably with some sort of normal/bell-curve distribution. So pick whatever physique looks buff enough to you, you won't make your character any less Dumner, whatever weight you pick.