Bigger HP can save more for big room because it will cool down the room faster compared to smaller HP which might not cool down the room to the required temp at all resulting the smaller HP to run in full load all the time.
Once the required temp is achieved, the load will be lower.

Interter do lead much to savings for my bill by using it to get 27°C constant temp. The motor will stop when it reached the required temp for both inverter and non-inverter type. The saving part is on getting the motor on/off to maintain the constant temp in longer hours. That's why the savings is noticeable if using inverter for longer hours to maintain 26/27 °C. It won't save much if inverter is to cool down to 20/21°C as it will be running almost on full load all time.

no difference on wiring for air-con. The difference is maybe the aircon is using another phase on 3 phase instead of using on the same phase on single phase.

nope, the number of cables to the aircon is still the same.

Install exhaust fan to blow off the hot air from the compressor area, or maybe change the compressor setting/location.

Nope, bathroom exhaust fan is too small to get the work done, i think u need bigger one like those in restaurant

I think it make some sense, if the room is small, the 2HP aircon won't run on full load all the time as compared to 1HP aircon, hence, it might consume lower electricity compared to 1HP.

Besides the room size, you need to check the thickness of the piping, and the diameter of the pipe as well.
If you calculate the room size and it requires 2HP aircon, but your diameter of the pipe is for 1HP aircon, then you can't use the 2HP aircon without changing the piping.

you got me.
you are right, u r the only expert here. I am talking rubbish, sorry bout that guys.