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Why do you think it's not reducing the captured length?

On Ethernet, the link-layer header is 14 bytes. The default IPv4 header, with no options, is 20 bytes, and the default TCP header, with no options, is 20 bytes. That's a total of 54 bytes, which is less than 80 bytes, so if you set the snapshot length to 80, you will see the entire TCP header unless there are a lot of options in either the IPv4 or TCP headers.

And the "Len=" in the output is coming from the on the wire length of the packets, not the captured length, so you can see "Len=" values bigger than the snapshot length.