Packing unity textures into atlases

Hello, I'm trying to create atlases, which will hold only textures that weren't scaled down, number of atlases doesn't matter. I'm using Texture2D.PackTextures() for that, but it scales textures down if they don't fit. To avoid that, I tried to sum areas of each texture and compare it to the sum of rectangle areas, but results always differ and I actually don't have any idea how this function computes that.

Any clues?