Looking for why XMLPARSE( DOCUMENT '<LargeXml>') worked and 'LargeXml didn't'

Hello all,

Had a problem that I solved, but I'm not sure why it worked. I'm looking
for what I can do to understand.

I had an 85k xml document I wanted to stick into an XML column in a table
with PHP. I was runing the php on the IBMi with db2_* using a prepared
statement.

Long story short UPDATE LIB.TABLE SET FOOXML= ? WHERE ID=? worked for small
XML strngs, but not a large (~85k) ones. The fix was changing my SQL to
UPDATE LIB.TABLE SET FOOXML= XMLPARSE(DOCUMENT ?) WHERE ID=?.

The error details I got from PHP were as follows:

Error Number:58004
Message:SQLSTATE=58004 SQLCODE=-901 SQL system error. It is a V7R2M0
system.

Is there any other tracing I can turn on on the PHP or DB2 layers to figure
out what happened. I'm ok with adding a call to the XMLPARSE UDF in the
future, I"m just curious to understand why the system couldn't figure out
how to parse and cast a large (but simple. No namespaces, attributes or
escaped characters)) XML string on its own but could for a small string.

Justin