I used this term recently, and I know what it means, but I never understood how it came to mean that. It's just one of those cliches that you never look too closely at. So I went out, and here's the answer I found.http://www.worldwidewords.org/qa/qa-bya1.htm.
It's a maritime saying (like a lot of our cliches). It has to do with the direction the wind is coming from on a ship, oddly enough. If the wind is blowing direcly into the side of the ship (the long end, not the skinny end), then the wind is said to be "on the beam". If the wind was coming from any direction in a semi-circle from that point (more towards the stern), it's called sailing "large". However, if you were sailing into the wind (which is technically possible by adjusting the direction of the sails slightly), then you were sailing "by the wind".
"By and large" sailing means you're sailing with the wind coming from anywhere. It covers the wind coming from all points on the compass. So it was basically a term to mean all-encompassing, or all things considered. So it's pretty easy to see why we say by and large to mean "for the most part" today.
Huh...you learn something new everyday.
That's the whole point of the New Things category. I figured that I would start documenting the new things that I learn every day. :-P
There, and I fixed that horrendous link.