> having more citizens should benefit a national economy

Generally that trend holds true - as long as migration is well handled. There are immediate infrastructure costs that need to be paid that the Country won't immediately see a return on.

Also governments aren't particularly profitable at the best of times, most of them are running at a deficit regardless of the number of people in their Country.

> The comment really applies to the US empire

Your comment is particularly ridiculous if your intent was to apply it to “the US empire” (where everyone in the territories has the right to move anywhere in the country).