As with the many different types of plug and socket in use around the world, different switch and socket mounting formats and dimensions have evolved in different countries and become embedded in the local electrical regulations. Customers also become familiar with locally adopted designs, and have expectations regarding the appearance of visible parts such as switch rockers.
Comparing the narrow abutting modular system often used in Italy with the wide spaced format popular in Germany, I suspect that a significant factor was the difference in socket outlet design at the time the modular principle was adopted. Although Schuko sockets are increasingly used in Italy, the local CEI 23-50 3-pin socket-outlet was historically the most common choice for domestic installations. This has approximately the same dimensions as a narrow switch rocker, and narrow rockers and front-mounted switch mechanisms were already popular in Italy and made by most major brands. Making each module only the width of a socket allowed maximum advantage to be taken of this similarity.
In Germany and German-derived installation practice, full-width rockers were historically popular, and Schuko sockets are deeper and wider than CEI types. Therefore it probably made more sense for German manufacturers to adopt a conventional socket-box outline as the module, and make other components and multi-module plates to match as this resulted in familiar rocker dimensions.
Each system has its own merits and demerits (as do the UK, US and other systems.) An Italian module frame allows many devices to be fitted side-by-side in a single box without providing electrical isolation for circuits in different bands (e.g. AV ports and power wiring). The German system by default (using a dividable or multi-compartment back box) provides separation and more wiring room.
Here in the UK we use neither of the above. Our system has the largest footprint of any in the world, again driven by the outline of our plugs which are wider but flatter than most others and the preference for switched sockets. We can fit three switch rockers in the area of one socket, compared to the USA where two sockets fit in the area of one traditional toggle switch. USA and UK practice traditionally shared the practice of exposed fixing screws, whereas European material favoured concealed screws and snap-on cover frames.
