Yeah, all SPC files will have a minimum of 0x10200 (66048 decimal) bytes, with potential to have additional bytes in the form of Extended ID666, or xid6 for short. xid6 holds tags longer than can be stored in the file's header, and tags that aren't present in the header at all. AMK (and afaik, any previous Addmusic) doesn't bother with xid6.
I don't think it's a huge deal that the full uncompressed ARAM is dumped to the file, due to it being only 64k. Makes it easier to read the binary too