IXFR size limit?

[Matthew Pounsett]

Is there, by any chance, a maximum size to the IXFRs BIND will send?  I've noticed an upstream server I slave from is being suspiciously consistent in the number of records it sends per IXFR (86,450 plus or minus ~10 records).  The upstream server is part of an appliance, but fingerprints as BIND 9.2.3rc1 -- 9.4.0a0.  

I don't see anything in the ARM for configuring a limit on the number of records per zone transfer, but I'm curious if there's something hard-coded somewhere...