Indeed, that was the point of using bmaptool: the actual used space is about 1/10th of the whole image, weighting around 400MB, and with bmaptool you can avoid writing a lot of zeroes to the SD card.

The data is more or less split like that:

224M usr 90M lib 67M var 11M boot 5,8M bin 3,3M sbin 2,9M etc

Of course with some customizations one can do a lot better, but this is just a straight Debian installation with documentation, locales, translations, manpages, etc.