According to msdn, the BC1 block compression format is for images with 3 color channels and 1 bit alpha channel and the BC4 is for images with 1 color channel and no alpha.
Using the same RGBA input image I would expect the output of this command
texconv.exe -f BC1_UNORM diffuse.tga
to be larger than the output of this other command
texconv.exe -f BC4_UNORM diffuse.tga
Since the resulting DDS should only have information on one channel, but both DDS have the exact same size. The only difference being that the BC4 compressed one appears red (as expected).
Is this the expected output?