Try running the
iotop(1m) command from the Terminal. This will give you a list of processes sorted by disk I/O usage. The "BYTES" field tells you how many bytes were part of the I/O operation (whether read or write).
This was also covered in a previous Super User question, here:
What's causing all the disk activity on OS X?
bitesize.d mentioned by someone else, I don't think its as useful because its purpose is specifically to track the size of individual I/O events per process. It's a debugging/investigative tool to see which processes are writing larger or smaller chunks of I/O at a time, and therefore which processes are making efficient use of the disk, not to see which processes are most actively writing the disk.
It can be used for the latter, but requires some extra steps. You have to tally up the histogram bins for the larger I/O sizes (value field) by multiplying the bin length by the size of the I/O operation. If you have a large number of processes performing I/O, this can get tedious really fast.
Another disadvantage is that
bitesize.d has to gather data before it displays results since it needs to compute the histograms. Therefore, for "online" analysis of disk usage, I recommend