You can have a powerful democracy with a small number of constituents because then an individual has a realistic capacity to rally his neighbors and make change when something sour is going on.

You can have a large democracy when the government's power is strictly limited to a small well-defined area of influence, because then most constituents can realistically remain informed about that specific area of influence.

You cannot have a large, powerful democracy. A large democracy with many powers can't resemble a direct democracy because constituents can't realistically remain informed on so many issues. But it also can't resemble a republic because that forces voters to choose between e.g. the party that promotes censorship and the party that promotes police militarization, without sufficient influence to affect the position of either party.

The problem is that more and more of the things people describe as "democracies" are large and powerful.