The doubled letter means smaller, and the ladder runs N, AAAA, AAA, AA, A, B, C, D. Two of those rungs left the retail shelf decades ago, which is why the sizes you can buy look like they skip.
I wrote up the whole thing: which battery codes carry the cell's real dimensions, which ones are just index numbers wearing the same costume, and why the moment you pick up something rechargeable you have crossed into a standard that counts differently.