Hi lemming,
If you follow the link you provided and click on the "samd21_sam_ba.bin" or the "samd21_sam_ba_genuino.bin" files, you'll find that the executable file sizes are both 6.35KB: ArduinoCore-samd/samd21_sam_ba.bin at master · arduino/ArduinoCore-samd · GitHub.
This is consistent with the fact that the Zero reserves 8K of flash for the bootloader itself.