The problem of win change in tcpdump packet capture


15:39:01.066209 IP client-ip > server-ip: Flags [S], seq 2676220664, win 65535, options [mss 1460,nop,nop,sackOK,nop,wscale 13], length 0
15:39:01.066247 IP server-ip > client-ip: Flags [S.], seq 3077872679, ack 2676220665, win 29200, options [mss 1460,nop,nop,sackOK,nop,wscale 10], length 0
15:39:01.066316 IP client-ip > server-ip: Flags [.], ack 1, win 5334, length 0
15:39:01.093054 IP client-ip > server-ip: Flags [P.], seq 1:381, ack 1, win 5334, length 380
15:39:01.093086 IP server-ip > client-ip: Flags [.], ack 381, win 30, length 0
15:39:01.093275 IP server-ip > client-ip: Flags [P.], seq 1:130, ack 381, win 30, length 129
15:39:01.093320 IP client-ip > server-ip: Flags [.], ack 130, win 5334, length 0
15:39:01.093354 IP server-ip > client-ip: Flags [P.], seq 130:133, ack 381, win 30, length 3
15:39:01.093395 IP client-ip > server-ip: Flags [.], ack 133, win 5334, length 0
15:39:26.093735 IP server-ip > client-ip: Flags [P.], seq 133:136, ack 381, win 30, length 3

excuse me, when the connection is established, the win is 65535 29200 respectively. If a large amount of data has not been sent below, how can the win of both sides become 5334 30