Help with OpenEXR HDR Texture Compression

I created a sky texture in Photoshop using 32-bit RGB, exporting to an OpenEXR format, and imported to UE4. It imports the colors just fine (more than fine, it looks incredible!), but the texture itself does not seem to compress properly. I set the texture compression settings to “HDRCompressed (RGB, BC6H, DX11),” but a single 2K texture uses 43.7 MB. According to Microsoft’s documents, a BC6H 4x4 texel block should only take 16 bytes, a total of 4.2 MB for a 2K texture.

Something seems wrong here, like my texture is not compressing. This is a PC game, not a mobile game, so what gives?

Also, my lighting build times increased dramatically when I started messing with this HDR texture. Lightmass, which used to calculate so quick, slows to a crawl. Is there some sort of bug with HDR textures we’re not aware of?