Are these proportional and/or similar? If so, please explain.

Note that

Every dimension of the largest rectangular solid is twice that of the "intermediate" rectangular solid

So....they are similar (and proportional)

And every  dimension of the largest rectangular solid is four times that of the smallest rectangular solid......So they are similar (and proportional)

But, by Euclid....two things that are similar to a third are also similar themselves....thus the "intermediate" rectangular solid and the smallest rectangular solid are similar to each other

So....all three objects are similar  (and proportional) to each other