Part of the size of the Sonnet firmware, I suspect, is that it has to include SCSI Toolbox emulation in order to allow booting of pre-OS X systems. By comparison, the smaller WiebeTech firmware just has to set up the OpenFirmware correctly in order to pass control to the OS X driver.
My limited experience with the DOS flasher was that it may require padding the image out to a size which is a power of two (or possibly a multiple of 128kb). That's what ahppended for me when I used it to flash the Wiebe firmware on a cantankerous card.