Quere keep using temporary files

Hi, we frequently run many query involving XML that use a smallish
temporary file, despite having increased local work_mem in that
transaction to 16GB. FreeBSD's top shows that the memory isn't actually
being used - it remains free. Basically many such queries are run within
a single transaction:

LOG:  temporary file: path "base/pgsql_tmp/pgsql_tmp92452.1079", size 166518
STATEMENT:  DELETE FROM "foo" WHERE ((col1, col2, col3) in (select col1,
col2, col3 from foo_xml_v2('<here goes xml-formatted blob>'))) AND
"foo"."col_id" IN ($1, $2, $3, $4, $5, $6, $7, $8, $9)

It looks like increasing work_mem doesn't help. Surely 16GB is enough to
cover all these small temp files?