Seems one of the dev of vbox was answering on this topic a while ago. http://forums.virtualbox.org/viewtopic.php?f=9&t=1822...
VD images files are typically mapped onto physical rotating media. These have high burst bandwidth but poor seek times (compared to SSD). The VDI format uses 2Mb pages for performance reasons. Dropping this to 4K to align it to the SDD driver technology would have a disastrous impact on real I/O performance (up to a factor of 10 slowdown say). Sorry, but this is a dumb idea.
This was not wrong 1 year ago.. now, I'm doing desktop virtualization on ssd laptop. I dont want my virtualdisk to grow undefinitively. I dont want spinning disk optimizations.