Although the appraisal of economical and commercial parameters relating to ship business is never trivial, it is rather intuitive that goods transportation by larger ships is more profitable, given of course the appropriate logistic boundary conditions. As far as oil tankers are concerned, it may be of common perception that a bigger ship represents a greater hazard. In order to gain a possible practical insight about the specific aspect of ship size influence on the relevant environmental impact, a Product, a Panamax, an Aframax and a VLCC have been investigated and compared in terms of oil outflow performance. As measurement tools, the three parameters defined in the IMO probabilistic methodology for the approval of alternative oil tanker designs have been exploited. As it can be expected, not only the size but also the internal subdivision has a significant influence on the ship outflow performance.@en