I found something strange in ext/pdo_mysql/mysql_statement.c
There is constant defined called PDO_MYSQL_MAX_BUFFER like this:
#define PDO_MYSQL_MAX_BUFFER 1024*1024
I've changed this to:
#define PDO_MYSQL_MAX_BUFFER 1024*1024*10
and recompiled PHP.
Now PDO loads whole 1.5MB from DB.
Hope that helps.