Wouldn't limiting power also mean limiting their effectiveness? A government (at any layer) needs to have a certain amount of power, else they're just civilians.

As for budget, a country needs money to do stuff; if they don't have money they can't do stuff. Stuff can range from having the world's biggest army (several times over) to providing free education to everyone (the great social equalizer IMO, as in social mobility).

As for your justice argument, it depends - if power corrupts, wouldn't giving more power to justice corrupt them as well? You see what's happening in the US with various law enforcement branches getting A Lot Of Money - militarization of local police force for example, meaning they have the means to apply more violence.

TL;DR, governments and justice systems need a clear description of what they can and cannot do, and checks, balances and consequences when they don't.

> Corruption within private companies is irrelevant, as the main ones to suffer from it are usually shareholders.

This ignores the vast majority of anyone involved in a private company - the customers. Or even the not-customers that are still affected by what a private company does (think e.g. pollution), but that's where as you say the law should come in.