If you have a look at this line in the script:
(file-png-save RUN-NONINTERACTIVE image drawable png_filename png_filename FALSE 9 FALSE FALSE FALSE FALSE FALSE)
And compare with the entry for file-png-save in the procedure browser (Help>Procedure Browser) the parameters are:
run-mode (do you want to prompt the user or not)
image the internal id of the image
drawable in this case it's the internal id of the only layer
filename is the name of the file to save the image in (png_filename)
raw-filename is the raw name of the file to save the image in (png_filename)
interlace use Adam7 interlacing? (= FALSE)
compression compression factor (=9)
bkgd Save background colour (=FALSE)
gama Save gamma (=FALSE)
offs Save layer offset (=FALSE)
phys Save resolution (=FALSE)
time Save creation time (=FALSE)
Update:
You wrote your update while I was writing this reply. I didn't deliberately do anything to crop the images, so I'm guessing that they are being cropped when they are loaded into Gimp. As for the size, that will probably be because I set the compression level to it's maximum (=9).