I think the code base released on sourceforge has the 4k bug fixed and therefore I don't know what it looked like previously so this is pure guesswork...
ioFTPD creates 4k of data for the .ioFTPD file and writes it out, then produces the next 4k, etc. If you don't do anything fast then the non-interlocked update that previously existed (guess) would complete and work fine. Thus a single user or lightly used server where nobody was in the same directory updating stuff at the same time might be able to survive and create larger .ioFTPD files. As with all race conditions it's luck on whether you get a corruption or not as you would need two writes to the .ioFTPD file to be going on at the same time with different data...
I know for a fact that I've created 16k .ioFTPD on 584u and couldn't get it to crash so it's not just size that is the problem so I assume it's the race condition writing.
P.S. I'm down to my last feature before releasing v6.0
My changelog includes 30+ new commands/features and I think you will all be very happy!