I just voted in Berkeley, ca - 5 min walk from my house and no one in line at 8:10am. So the problem of long lines isn't universal.

Are these other countries comparable to the US in terms of scale and population?

Yes, of course. Long lines to vote is not a function of the size of the country, it's more likely to be a function of the number of people and the number of available polling stations in a given area. There are very many large cities around the world that can handle voting without the excessive lines seen in the US.

It's not the overall population that is the problem: millions of people are not voting at any one location.

No but elections are managed by the state and local governments in the US - those are comparable to other countries.