Unfortunately, I don't think anyone knows the answer to this question. There
are some suggestions that it is to do with either the total amount of data
in the record, or in the data source, or to do with a limit on some buffer
that Word uses during mailmerge - for example, it may try to cache a certain
number of records and start truncating fields when the buffer fills. One of
these days I may attempt some better controlled experiments to try to find
out, but I'd rather MS just made it work or told us what the limitations
are.

Peter Jamieson