If you turn on the chunk markers on Rei's Minimap (Surfacemap options -->show chunk grid should be "enabled"), you can see where the chunks are.
As you can see with my quarry here wasn't really placed in a good spot - it's keeping 2 chunks loaded. The chunk with the quarry in it, and the chunk infront of it (which has the chest and power stuff). But still it's only keeping 2 chunks loaded. Usually when I place them, they are only keeping one loaded. With this smaller default size quarry, you can have the entire quarry within one chunk, or as Dewey said, at worst you've placed it in the intersection of 4 chunks and are keeping 4 chunks loaded.
The larger quarries can keep 25+ chunks loaded for one single quarry. You consider that some people had multiple quarries too, that's a lot of chunks always being kept loaded.