proc_open, fwrite, 16384 bytes magic

Hi everyone, I've ran into a rather strange problem with Apache 2.0.x, PHP 4.4.0, FreeBSD. When I proc_open qmail-inject, the first 16384 bytes from a buffer are accepted perfectly, which makes it an exact 16 KB. If a buffer is larger, the rest is discarded. No errors appear anywhere - and I think I've looked just about everywhere I could think of by now. This problem does not manifest itself on a different installation of Apache 2.0.x, PHP 4.3.8, SUSE Linux, but tested against postfix, up to sizes of 95 KB. I've searched Google all over, but could not find any hints. My best guess so far is that I'm missing something in configuration directives, or it's a qmail problem - although, with qmail-inject fed via cat, everything works just fine, regardless of size. I would be grateful for any hints. Thanks!