https://bugs.winehq.org/show_bug.cgi?id=42412
--- Comment #23 from Bruno Jesus 00cpxxx@gmail.com --- If you compare both logs (1488 and 1500 mtu) you will see that they are nearly identical, most reads are the same size (1448 bytes).
If then you select the FTP download connection after the RETR for the file and select menu Analyze -> Follow -> TCP stream on both connections and then save both files as RAW and compare them the result will be 2 identical files. I also extracted them (since it is a LHA file) and SHA1 sum both extracted file contents (5 files), all matches. I double checked that I compared the OSX files.
The tcpdump logs are less informative for me, the weird part is that the 1500 byte MTU seems to be really respected as all reads are 1500 bytes, but the 1488 MTU works in packets of 770 bytes, so the log size is doubled. I don't know where this 770 number comes from.