Mod_proxy Chunk minimum file size

I am using Apache HTTPD 2.2.17 + Mod_proxy + SSL on Ubuntu as a gateway. Everything works fine but when I send a request with Transfer Encoding: Chunked, it converts(de-chunk) back if the file size is too small.

I tested with a 43k works fine. Tested with 16K and 5.5K it's getting converted as a request with 'content-length'.

My conf file:

<IfModule mod_ssl.c>
NameVirtualHost *:443
<VirtualHost *:443>
  ServerAdmin *
  ServerName  *

  DocumentRoot "/var/www"
  ErrorLog /var/log/apache2/error.log
  LogLevel warn
  TransferLog /var/log/apache2/access.log

  SSLEngine on
  SSLCipherSuite ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL
  SSLCertificateFile /etc/ssl/certs/*.pem
  SSLCertificateKeyFile /etc/ssl/certs/*.pem
  SSLCertificateChainFile /etc/ssl/certs/symantec_intermediate_ca_bundle.pem

  CustomLog /var/log/apache2/access.log combined

  ProxyPreserveHost On  
  ProxyRequests Off
  #ProxyIOBufferSize 1024
  #SetEnv proxy-sendchunked 1
  #ProxyPass / http://localhost:9443/ flushpackets=on proxy-sendchunked=on
  #ProxyIOBufferSize 1024
  SetEnv proxy-sendchunked 1
  ProxyPass / http://localhost:9443/ iobuffersize=1024
  #ProxyPassReverse / http://localhost:9443/
  #ProxyPass / http://localhost:9443/ flushpackets=on
  ProxyPassReverse / http://localhost:9443/

</VirtualHost>
</IfModule>

httpd 2.4.1 too got the same issue.

Any help would be appreciated