When you zoom in, the scale of the map increases (it gets larger). A large-scale map covers a smaller land area than a small-scale map. A map that is zoomed in to a small town is large scale, while a map of Canada is small scale.

This is rather counterintuitive. In fact, it confuses many people.

Explanation: Scale is a ratio. 1:10 means that every unit on the map represents ten units in the real world. Or in this case the map is one-tenth the size of the real world. Because its a ratio it doesn't matter if the units are centimeters, miles... anything. If you have a 1: 100 000 map, the map is 1/100 000 the size of the actual place. 1/100 000 is a much smaller fraction, a.k.a ratio, a.k.a scale than 1:10.

Do you have a headache yet? Let's move along, shall we?

Just remember this...

Larger scale equals greater detail. That's all you really need to know.