LwIP Configuration problem

I use the simple socket server project which provided by NiosII IDE.

I modify the sss_handle_receive( ) in simple_socket_server.c and use sss_handle_receive( ) to receive data which sent by PC.

The sss_handle_receive( ) use the code as below to receive data .

rx_code = recv(conn->fd, conn->rx_buffer, data_length , 0);

For optimizing the LwIP Throughput , I use the NiosII IDE to set the value of TCP_WND .

When I set the value of TCP_WND for 32768 , the data I obtained is smaller than data_length .

But it is normal , if the value of TCP_WND is 2048 .

How can I do to solve the problem ?

thanks~