Direct injection has the entire combustion chamber located inside the piston crown, thereby allowing faster more complete combustion with the heat concentrated within that space making it more efficient. Also, since the flame front is more controlled within the piston top combustion chamber, igintion timing is more precise and quicker, allowing higher top rpms and more torque and power. Also, multiple injections can be programmed through the more precisely located injector for even more fuel efficency and power output.
Actually, I am amazed that Kubota has not gone to DI on their smaller diesels primarily to meet emission regulations, the fact that they manage to meet current emission regulations with their IDI designs means that it is an especially good design.
Also, due to the fact that DI engine combustion chamber is ovally located inside the top of the piston, the minimum combustion chamber size that DI will operate in is about 400-500cc, since anything smaller diameter than that will result in raw fuel from the injectors hitting the piston combustion chamber wall prior to TDC and destroying the piston with intense heat from the raw unatomized fuel buring on it.