I did some more research and i found this sentence:
"Combat range: 750 km (405 nmi, 466 mi)at sea level, 4,400 kg (9,700 lb) weapons and two external tanks"
If it is true that probably find the problem. This combat range includes extra fuel to get 750 km in total.
SU-25 has 3600 litres of fuel in internal tanks. For example SU-27 has almost 12 000 litres in internal tanks and uses more fuel efficient turbofan engines (offcoures without afterburning) and the range of SU-27 is 1390 km at sea level. Offcourse fighter vs bomber, you can't compare it. Just see those numbers A-10 - 4850 litres and capable of air refueling, F-15C (max ferry range) almost 31500 litres of fuel (about 7800 in external tanks)
Well imho it seems SU-25 has relatively low internal fuel capacity against the other aircraft and plus it has less fuel efficient engines. That's for real life, not for the game.
And i know that this is much more complicated than just ammout of fuel and type of engines, but still 2850 kg of fuel is not much.