I did some logs on my proxy test and compared results with wireshark trace at some random point in time (t=511sec)
And numbers match exactly between logs and wireshark.

This is a log line from my test proxy:
time: 511s, bytesSent:5571760, down:{ SND:478720 OUTQ:280480 } up:{ RCV:5109117 INQ:3837047 }
SND is SNDBUF, RCV is RCVBUF, OUTQ is SIOCOUTQ, INQ is SIOCINQ/FIONREAD.
down is the link between wget and proxy, and up is the link from proxy to node on localhost.

At the same time in wireshark the up link has 9409056 bytes ACKed, down link has 5291529 bytes ACKed, or 9409056-5291529=4117527 bytes got accumulated inside proxy process. This is exactly the same number of bytes that's stuck in socket queues on up+down links (280480 + 3837047 =4117527).

From logs it also looks like when INQ and OUTQ reach values close to SND/RCV then in wireshark I see TCP packets with window size = 0 to stop any transmission.
Perhaps, if I set SND+RCV to 64K then total buffering should not exceed 128KB inside proxy buffers.