Just compare the size of a raw file with that of an interpolated file,
What is an "interpolated file?" That's not just a thing by itself. To be sure it is interpolated, you have to save in some specific format like jpeg or png or tiff or whatever, all of which have different baggage they come with confounding variables (compression amounts, or overhead data) that makes it difficult to compare the file sizes with certainty. Which format are you choosing to do your comparison with, and how are you solving the ambiguities involved in inherently different formats?
For example, the closest I can come up with would be to convert a RAW to photoshop and then save it as a lossless-ly compressed PNG file. When I do that, the file gets SMALLER by about 1 megabyte. Granted, I am going from 12 bit to 8 bit, but... (three channels 8+8+8 = 24, which should be twice the size of one channel @12)
Which is totally ambiguous. I can't interpret that either way without knowing the details of what sort of compression algorithm my Canon RAW files are using, if any. Some compression methods would become MORE efficient with 12 bit images (like dictionary methods), and others would be less efficient (like PNG style compression). Or if they use none, then that leads to a different conclusion.
Where did you get the information that raw files are already debayered? Even without a reference, the evidence is there in the file size that they only contain one channel of data.
Where did you get the information that they are NOT already debayered? Your file size evidence is less than convincing (see above). If you have some other reliable reference to how proprietary formats work, then that changes things.
Did you sit down and decompile and reverse engineer them in your free time? Do you work for Adobe or something?
If so, I'm super curious to hear a rare inside scoop on how some of these work exactly. If not, then you're inferring, just like I am.