Why do my shadows look worse if I make my ground larger?

I’m not too sure how it works, but let’s say I start off with a default plane with a cube on top. Lightmap it, and you get nice smooth, crisp shadows. Now, scale the plane to about 20,1,20 and redo the lightmapping. The shadows are choppy, jaggy and terrible to look at.
Why does that happen and how do you fix it? I’ve tweaked pretty much every shadow setting, even brought in a plane with over 15000 faces (lol) which didn’t make a single difference. No matter what I do, the shadows look worse if the ground it’s being cast on is too large.

Shadow uses texture, texture is scaled along the plane. The larger the plane, the more texture gets scale and pixelated. You can increase texture size up to some limit. Then, you need divide plane in many different parts, and lightmap them independently (different texture for different part).

Its because lightmap can not tile.