In early sailing jargon, by was “by the wind,” and when a helmsman was ordered to fill the sails he was told to steer “full and by.”
This required great skill and was called steering small.
A less experienced helmsman might have been told to steer large with the order “by and large,” which meant use the wind but don’t fill the sails.
This is how “by and large” came to mean not quite true, but close enough.
We describe something approximate as “by and large”.
