Manus wasn't actually evil. Wasn't it Gough who said "Seduced by a dark Serpent or not, they awoke that thing themselves...and drove it mad." And in Dark Souls 2 many hexes state that the Dark is "a thing beautiful, a thing serene". Although, I'd say Nashandra is definitely evil, but that might just be because Manus probably died with a lot of hatred for humans in his soul.
Gough did say that actually which leads me to believe the people of Oolacile were dicks. I think Nashandra herself is oddly a victim of the curse of want. Her soul states she desired power to overcome her fragility. Though, what is creepy is that her human form looks a LOT like Dusk of Oolacile.
And that makes sense, because being the weakest of the shards, she had to disguise herself as a human. And Dusk was one of the few humans Manus ever saw.''
There's probably only one thing that still confuses me. Why are Vendrick and Nashandra so big? I mean, Gwyn was big too, but he had a Lord Soul. Is it because Vendrick "made the power of the Giants his own"? Or is that referring to the "Prize" he stole from them? Speaking of which, I think the Ashen Mist Heart could be it, since it can look into memories. And if Vendrick made "magnificent findings on souls" that would mean he studied Soul Memory using the Ashen Mist Heart.
There's probably only one thing that still confuses me. Why are Vendrick and Nashandra so big? I mean, Gwyn was big too, but he had a Lord Soul. Is it because Vendrick "made the power of the Giants his own"? Or is that referring to the "Prize" he stole from them? Speaking of which, I think the Ashen Mist Heart could be it, since it can look into memories. And if Vendrick made "magnificent findings on souls" that would mean he studied Soul Memory using the Ashen Mist Heart.
I used to think it had something to do with Vendrick's claiming the Giants' power for himself; however, I think honestly it has something to do with the amount of souls he had acquired over the course of his life.
So...If you collect enough souls you become bigger? 
Either that or it's a gameplay-story thing
We may never know...
Unfortunately! 