One of the interesting things about that, is that the reason that the Bulgarian capital is at the edge of the country, and not nearer the center, is because Bulgarias original borders incorporated modern Macedonia, when Russia originally backed the creation of Bulgaria as its Balkan client state, as agreed to at the Treaty of St. Stefano.
Later, the other European powers, in particular Austria, which was officially a Russian ally at the time, objected to the Russians having this massive client state carved out of the ailing Ottoman Empire, and the treaty was amended and these territories went back to being an Ottoman protectorate.
Sofia remained the capital, of was now a much smaller state.
Bulgaria under the Treaty of St. Stefano
Ok, so maybe its not so interesting but there you go. Regardless, all of these regional powers had a big fight about this in 1912, and the territory ended up in Yugoslavia. Looked at in one way this is all part of the conflict which eventually triggered WWI and by extension WW II.