With people from the modding community coming out trying to disprove the leak as an amateur job, what I'd love to see confirmed is whether or not there's actually a 2mb limit on the costume pieces. That seems to be the clincher here. Very specifically, I'd like to know if that's total, for the mesh and the texture, or if it only counts the mesh.
I was on mobile earlier when joking about the cube thing, but the ease of the cube does prove a point. If you're not familiar with how games translate models, typically modeling programs present objects as having rectangular faces, but video games read them in as triangles, so while a cube is simplistic and has 6 faces to us, to a game, it has 12 tris, which is size and processor. So, Mallow hat, for instance - the details seem to be too pronounced for a lot of it to just be normal mapping to provide depth, so a lot of that is likely a real mesh, and for it to have such smooth curves, they would have had to subdivide a cube several times to get nice smooth rounded edges, which even one subdivision (turning one cube into 4 faces per face) ends up making 8 tris per face (times 6 faces = 48 tris), so it compounds very quickly. I'm not an expert but I'd say that Mallow has several hundred triangles in just one of his puffy... puffs. That's pretty sizeable.
Then we have to consider the texture. I have no idea how big a texture is for an average mii costume, but they can get out of hand really quickly.
Obviously I'm not active in the modding community, but I guess it's possible that they could have somehow cracked the compression formula after all this time, but it seems like the first move a normal, sane individual would make is just announce that they made mii costume modding possible and not "leak" it to /v/ for no other reason than to stir up ****.