I've written a program that extracts all data from .szs files (found in supermariosunshine for example), get it with source and documentation from
Post to this thread if you find out what's stored in all the files that are generated from the szs/rarc files :-) Some wild guesses by me:
.bmg: Messages/Subtitles. Contain lots of text.
.bmd: 3d model data?? Contains "VTX1" blocks which could stand for "Vertex". However, most of these files are not too big. The "map/map" subdir of the szs files in "data/scene" contains a map.bmpd which is quite big (nearly always > 1mb), this could be the main level model.
[edit: fixed link :-O ]