Simple. Longer wheelbase translates into more space for a bigger tank.
The only way to put a bigger tank in the FJ would be to sacrifice ground clearance, or to put the tank behind the rear axle, with the inherent risk of fires in case of a rear end collision.
The best answer to deal with this issue is to drastically improve fuel efficiency, by equipping the FJ with a different engine. One option would be a small-displacement turbo-charged gasoline engine, similar to Ford's Eco Boost technology. A 2.5 l turbo could easily deliver ~250 hp while cutting fuel consumption by 15-20%. Another option could be a hybrid, although this would help mainly city mileage, with little impact on highway. Finally, Toyota could bring the 3.0 l turbo-diesel currently available in LC Prado and Hilux in other markets. This would give >25% increase in range, although the driving dynamics with a 170 hp engine may not be acceptable for some "lead-footed" American drivers.
We have after market additional tanks available for the FJ in south Africa. I have it on my vehicle.
Total fuel is 150 litres.
So getting about 1000km to a tank.
If I'm not mistaken the original tank is 73 litres and the additional tank is 77 litres.
Fits under the rear loading area of the vehicle.
Would anyone like pics?
Aftermarket fuel tanks that go in the space behind the rear axle are very common in Australia. Probably because many of our trips are more expedition style runs where you need to be able to travel 1000km between refuel locations, not hardcore day trips.
There are a few companies here that make the tanks, usually in 120L sizes such as Long Ranger and Long Range Automotive. They have thier own gauge and pump (the pump transfers fuel from the auz tank to the main tank)
I've got a 120L Long Ranger tank in mine - see bad pic below.
I'd hate to think of the cost (especially transport) to get one from Australia to the States though. Over here ARB is an agent/distributor for the Long Ranger brand, so you never know, might be worth asking an ARB store in the states.
From all that I have read, those aux tanks are not street legal in the US. Saw a company that made one that was an additional 19 gallons, went behind the rear axel and would have been great, but it was very costly and not street legal.