A few comments:

1) I had thought you wanted a "droplet" to give you the drag-n-drop ability. If you don't want this, then shell scripting is definitely the way to go.

2) The XMP specification states that 2kB of "padding" should be added. This is probably the bulk of the difference in size between XMP and IPTC. There is an API option to disable the padding, but I have not provided a command-line option to do this. So your best option is currently to convert to IPTC as you are doing.

3) I don't see why you shouldn't be able to do the resize and save as JPEG without having to go through a BMP. You could also pipe the output directly to exiftool to remove all intermediate files (see the few last posts in this thread for an example.) You have installed "convert" on your Mac? You must be a power user... (ie. Unix background) :)

- Phil