900 tons seems pretty high, especially for a Japan that was running low on supplies near the end of the war. However, it's not an unreasonable number. For instance, the U.S. stored the same number (900 tons) of explosives on Parry Island:
http://www.realwarphotos.com/wwii-famous_battles
Probably Ryukishi was just trying to find a big number to write down. He guessed a bit high, but eh, so what. Even 100 tons would have been more than enough to blow the mansion sky high.