Lighttpd

See:
http://redmine.lighttpd.net/repositories/entry/lighttpd/branches/lighttpd-1.4.x/src/mod_proxy.c
Function:
static int proxy_demux_response(server *srv, handler_ctx *hctx)
Line:
lien 655: buffer_prepare_append(hctx->response, hctx->response->used + b);

Too waste memory.
int buffer_prepare_append(buffer *b, size_t size) expands the content of the buffer, not set. The argument size is a "incremental" quantity, not the "absolute" quantity. See the srouce.
So line 655 should be changed to:
buffer_prepare_append(hctx->response, b);
I have had a test, and it works OK.