Window size reflects the maximum number of packets that can be sent without waiting for positive acknowledgment. Large TCP windows improve TCP/IP performance when large amounts of data are in transit between the sender and receiver. In typical TCP-based communication, the maximum window size is usually fixed at the onset of connection and limited to 64 kilobytes.
With large window support, you can dynamically recalculate and scale the actual window size by using a TCP option as needed during longer sessions. With this option, more data packets are in transit on the network at one time, which increases throughput.
For more information about large TCP windows, see RFC 1323, "TCP Extensions for High Performance."