Increase SPI transaction speed?
Because the data is getting buffered somewhere with the larger packets?
How is the SPI configured and to what is it connected? 


As a general rule this kind of this is not best done this way. Are you actually using this code to do something or are you just trying to instrument the SPI capabilities? What else is running on the board and how is your kernel configured. What's the processor speed actually set to and what priority is your little app?

If you've got some kind of quasi realtime requirement then I'd suggest looking at

a) DMA (offload transmission from userland buffers)

b) Xenomai 

c) (for the madly keen) the PRU 

Hope this is not a homework question! Here are 2 links to get you started on A.

https://stackoverflow.com/questions/3333959/mapping-dma-buffers-to-userspace
https://groups.google.com/forum/#!topic/beagleboard/UPbU2WoVzVI