Well, all the large empires were corrupt at the end of their life, and all have successfully transformed.

There is no gene of corruption - it's rather a matter of prevalent culture, informed by socioeconomic factors. As these factors change, the culture has to change as well, even if this is painstakingly hard for local culture carriers.

By the way, it's remarkable that some of best levels of wellbeing and democracy are achieved by small Nordic countries. So the small size isn't a guarantee of happiness, but smaller countries are easier to govern in right direction.