Shell Script Disk Image Size

I am trying to figure out what the three numeric fields mean for an disk image in Crochet.
This is from the NewBoardExample and it offers no explanation of the values.
IMAGE_SIZE=$((1000 * 1000 * 1000))

When I increase the image size in the GenericI386 board file script it generates an image. Unadultered it fails.
IMAGE_SIZE=$((600 * 1000 * 1000))
When I increase all values to 2000 it builds OK. I could try values >1000<2000.
I tried 1000*1000*1000* but it failed. IMAGE_SIZE=$((2000*2000*2000)) works.

I know I am increasing the image size to 2000MB which is overkill but why are there 3 values? Why is the first smaller?
Alot of builds seem to use 1000 * 1000 * 1000.

IMAGE_SIZE is in bytes, 600 * 1000 * 1000 would be around 600 MB. The numbers themselves have no special meaning. It's just a calculation.