The maps were very accurate in terms of latitude (north/south position) which meant ships could find a particular port or harbor even if they had only a rough idea of their longitude (east/west position) by sailing along the latitude lines until they hit land. How big it made a particular piece of land look was irrelevant.

It's not well known, but maps started to become super-accurate in terms of longitude as well in the 1670s or thereabouts. Astronomers had observed that Jupiter had eclipses of its moons about a thousand times a year, and thanks to Isaac Newton and others they were able to predict the exact times (in Greenwich England) of these eclipses a few years in advance and publish them. Observers around the world with good telescopes could view these eclipses and note the time differences between their observations and the published Greenwich times and use this to precisely calculate their longitude. It was not really possible to do this reliably on board a sailing ship because of the difficulty of keeping a high-powered telescope trained on Jupiter on a pitching and rolling deck (although many attempts were made) so this led to accurate maps but still not fully accurate navigation until the perfection of ship's clocks a century later.

The map is extremely accurate in the sense that a course plotted on the map can be followed by following a compass course. The same does not apply for earlier maps, or by far most modern maps that are designed to match real-world sizes.

The reason the Mercator projection became the standard is that it was, for a long time, the only map you could use to safely navigate long distances, especially across oceans.