I haven't looked at the deep internals of transaction logging in some time and my brain is frazzled at the moment from a recent total loss of my main computer (thankfully able to restore with minimal loss) and a client that is tits up at the moment. But I believe that there is something other than a one-to-one relationship between the tlog "size" and the amount of data pages that can be affected by the given tlog chunk. So a few hundreds of K of tlog could be (possibly) generating 5MB/sec dirty database page writes.