I don't necessarily see a sweeping trend toward smaller tanks. For example, my first new car was a 1980 Mustang Ghia with a 200 c.i. inline six (remember the era before you start teasing me mercilessly); I seem to recall that its tank held somewhere around 12 gallons, and it was hard pressed to get 18 mpg, for a cruising range of less than 200 miles unless you ran it bone dry.
It seems that automakers shoot for somewhere around a minimum of 250 miles to a tank, using the EPA mileage figures multiplied by capacity.
A couple of nice things about not having a huge tank: When I travel a lot for work my Jeep sometimes goes only 100 miles or so in a month, so I don't have to worry about the fuel breaking down if I don't get around to topping off the tank, even though major fuel breakdown is fairly unlikely. Second, gasoline formulations in some markets are slightly modified to reduce emissions as temperatures fluctuate; in other words, the "winter formula" for fuel is slightly different than the ones used in other seasons. A smaller tank might make a slight difference in the quality of the air you breathe, and I'm all for breathing better air.
But again, those two advantages are a bit of a stretch...in reality, I think that manufacturers strike a compromise between weight, range and the ability to fit a tank within the vehicle.