It seems as if MySQLdb is restricting the maximum transfer size for SQL statements. I have set the max_allowed_packet to 128M for mysqld. MySQL documentation says that this needs to be done for the client as well.
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            You need to put max_allowed_packet into the [client] section of my.cnf on the machine where the client runs. If you want to, you can specify a different file or group in mysqldb.connect.
                  Martin v. Löwis
                   2010-10-29 11:51:19