This error indicates that you have attempted to download a file that is too big to fit into the available RAM.
Things to try:
Check that you have enough RAM to contain the file that you are attempting to download.
If you are downloading multiple images, check that you have freed any images that you no longer need. You can view the currently downloaded images using the imgstat command, and free any unwanted images using the imgfree command.
If you are downloading an entire disk or ISO image (e.g. for booting with MEMDISK), then consider using a SAN boot instead.