Yeah, fish is kinda big across the whole country. In fact, Italians tend to be surprised by how fish is not particularly valued in a lot of other countries. There are geographic and historic factors in play, obviously: Pisa was one of the "Maritime Republics" city-states at one point ( http://en.wikipedia.org/wiki/Maritime_republics ) so they clearly know their sea; nearby Tuscan cities tend to be more meat-oriented.

Among other factors, I'd say that a lot of "pasta sauces" are entirely based on vegetables and prepared from fresh ingredients, something very different from processed stuff you buy in jars.

I don't think portions are a factor -- they might be smaller, but even the smallest meal will have several different types of food (pasta, meat or fish, vegetables and then fruit). Single-portion meals are very rare.