Fighters don't stop obaying the laws of physics just because they are stealth, and you need a certain amount of size to be efficient in terms of range, payload and upgrade potential.
For an island nation like Japan, range is even more important because there are far less available and suitable bits of land for them to build air bases on. In addition, there is little to no strategic depth for island nations, so if they wait until the enemy gets over their beaches before they can intercept, then the enemy would already be within weapons range of any and every target they might want to hit. A large land based nation like China or the US can afford to allow enemy aircraft to penetrate a little over their coastal regions because even then there will still be vast swaves of areas the enemy cannot hit. That is simply not an option for the Japanese, which is why they need to intercept and engage hosile air units way out at sea in order to make sure vital areas/targets are not hit. That is why the Japanese F2 was made considerably bigger than the F16 it was based on.
The main reason the Japanese design is so tiny is the same reason the British Replica was so tiny - they were envisaged and designed as pure tech demonstrators with little or no intention of ever being made into flying prototypes never mind production aircraft.
For tech demonstrators, the only considerations are speedy and cheap production, which is why the demonstrators tend to be small, just look at all the American X-planes. With the exception of the likes of the X23/22 and X32/35 which were designed and built to fulfill tenders for real combat aircraft, all the pure experimental X-planes were pretty small.