How are Kuwait, Luxembourg, Singapore, and Monaco countries?

There's no limit on the size of a country, they can be as large or as small as they want. Luxemburg and Monaco are left-overs from the old European feudal order. There was a time when dukedoms and princedoms were independant from kingdoms and if a king wanted to attach one of them to their country, they had to marry the relevant great duchess or go to war.

Singapore is a left over from colonial times.

All these countries (and even the Vatican) are doing quite well for themselves and do not get into trouble with their neighbours, so why does this bother you?

Maybe they think America or Russia are too large. What would your answer to that be?

They come in all sizes like animals - a whale to a tiny mite.

Of the 4 you listed Kuwait is biggest by far at an area of 24280 km² (=9375 miles²). Next comes Luxembourg, all of 999 miles² (=2587 km²).

Hong kong & Macao also could have become city-states like Singapore but China claimed them.

Over the course of history nations have tended to get bigger, with larger ones gradually absorbing the smaller ones. The small ones remaining, at least in Europe, tend to be the surviving ones which didn't get absorbed - often because they were on the border between two large ones, neither of which wanted the other to have the territory.

Singapore is only a tiny red dot but well managed city state.