It's probably limited by weight rather than volume. You could pack them in like sardines till the weight limit is reached. Of course they wouldn't look too good when they arrive. The cargo section isn't pressurized.
Ok - I'm still looking for the drawback...
One reason why a lot of cargo airlines like the 747-400 is because it has a great lifting capacity, and some say even better than the A-380 once the 747-8I comes in service.