comparing weights of irregular shapes

I am trying to produce a graphic for a popular magazine showing how, through overfishing, various species of fish are now physically smaller than they were 50 years ago. My notion was to have an outline of a fish–a tuna, say–and within it a smaller outline showing the smaller modern size. In the 1950s tunas weighed an average of 163 pounds, today 82. Shrinking the larger outline by 50% is clearly wrong, but what is right? (To complicate matters further, I’d like the tuna to be proportional to other pictured fish–a 278 pound marlin, for example.)