Command Block Textures forced to 16x16

I've been working on some custom texture packs for Minecraft, and I recently discovered that we can /give ourselves command blocks.

Since my textures are 128x128, I decided to attempt to create 128x128 textures for the command blocks as well, using an extracted vanilla texture pack as a base.

I discovered two things: First, you can only modify textures for the command blocks if you install the texture pack as a global resource (not a big deal, but rather odd) and second, it makes no difference what resolution you set your textures for the command blocks to, because Minecraft PE always forces the resolution of the textures down to 16x16.

Is this a bug, and, if not, could someone please explain why/how I can make the textures fit?