TCP Slow Start / 14KB Rule
The first response packet will be 14KB. This is part of TCP slow start, an algorithm which balances the speed of a network connection. Slow start gradually increases the amount of data transmitted until the network's maximum bandwidth can be determined.
In TCP slow start, after receipt of the initial packet, the server doubles the size of the next packet to around 28KB. Subsequent packets increase in size until a predetermined threshold is reached, or congestion is experienced.
TCP slow start gradually builds up transmission speeds appropriate for the network's capabilities to avoid congestion.
Congestion Control
As the server sends data in TCP packets, the client confirms delivery by returning acknowledgements (ACKs). However, the connection has a limited capacity depending on the hardware and network conditions.
If the server sends too many packets too quickly, they will be dropped. Meaning that there will be no acknowledgement (ACK). The server registers this as missing ACKs. Congestion control algorithms use this flow of sent packets and ACKs to determine a send rate.