I've never been aware that Italians can understand Portuguese very easily...
Anyway, that's called asymmetrical intelligibility. Basically, a French or Romanian speaker would have a much easier time understanding another Romance language than a person speaking a different Romance language would have trying to understand any of those two. It's because of "closed" vowels and different article grammar in the case of Romanian and because of too many transformations and "swallowed" endings in the case of French.
Had the Dalmatian language not died, Romance languages in Europe would still be a dialect continuum. As in, from one end to another, each language would be understandable to its neighbors but less and less understandable as you go further away. As Dalmatian died early on, complete connection was broken, and only Portuguese -> Spanish -> Catalan -> Italian are still a dialect continuum. It used to be Portuguese -> Spanish -> Catalan -> Italian -> Dalmatian -> (Aromanian*) -> Romanian.
*= if you consider it a language and not a dialect
BTW, if we go by Wiki, Aromanian and Megleno-Romanian are languages and not dialects so we here understand 3 romance languages easily and one partially (Spanish).
Anyway, from my experience, in Romance languages, the more central you get, the easier it is to be understood by everyone and the harder it is to understand the others. Which is why Italian and Spanish are understood by almost all (can't comment on Catalan since unfortunately not enough people I know have too much of a contact with it), yet they have a harder time understanding peripheric Romance languages (like French, Romanian).
BTW, for me written Catalan is completely understandable (don't think I've heard it spoken too much though), but then again I learned to speak reasonable Italian (yes, without even trying, in two months of staying there

) so I'm probably more experienced in this than your average Romance-speaker.