The distance between the meridians decreases as you near the poles because of the lowering circumference of the Earth as you near the poles. To put it in a simple way, the Earth is a rounded body, elliptical, and the meridians are stretching from north to south, as they approach the equator the Earth is wider, so the distance between them become bigger, but as they approach the poles the Earth becomes narrower and they are coming closer and closer to each other until they finally all merge into one dot on each of the two poles.