Hippynerd:
Im not sure about the mega, but the 328 based units seem to be limited to 500ma at the 5v pin. My guess is that 1.3a is going to too much, even for the mega. I think that the 5v pin limit is from the specific voltage regulator that is used (the part right next to the external power socket.)
That and when using USB power there is a 500ma thermofuse that limits the maximum 5V current one can draw.
Lefty