Try running the vboxinfo plugin on your .elf file and look at the memory segmentation. Most likely you will see a range starting at 0xE0000000 which is for the virtualbox hardware device memory (vga, pci, etc). The imagecopy plugin will 0-pad your raw dump to be large enough to include those memory segments. This won't happen when you convert formats like crash, hiber, etc into raw because those don't contain the hardware memory ranges to begin with. A bit more info is in the meta-data section of
https://code.google.com/p/volatility/wiki/VirtualBoxCoreDump.