It is not so much the size of the country, but the efficiency at which they managed to limit people's movement. Don't forget that the government has a controlled social point system with face detection in place (that e.g. penalizes by limiting peoples' access to trains, airports and other possibilities for traveling, limit access to schools, or any other aspect of daily life): when the government says stay inside, people will fear the consequences as any disobedience can really impact their long term future comfort and life. So I don't think it is impossible. More democratic countries just cannot achieve that level of lockdown: for example, in Belgium, when the country went in lockdown, people organized lockdown parties on the last evening...In addition, I see that here in Poland people are much more isolating (restricting themselves more than the government does), but the explanation rather is in the distrust of the healthcare system: whatever you do, do not get yourself into a situation where you need to go to a hospital. So maybe such aspects also play...
But I honestly also don't know... But we can also look at South Korea, that also managed very well to limit the growth...