Reason why?

1)  prospective; shape and forms of the line and the way we see it.  As well as form and composition on how the artist organized the dimensions, shape and structure. Also creating space through perspective.

2) The boat that is larger is closer to the individual.  Meanwhile the boat that is smaller is farther away from the individual.

3)  The artist visited Italy a lot and could have liked how the ruins where.  Also could be the history or the beliefs the artist had.