1E, 2E, and 5E have one map scale, and 3E/4E have another. In 3E, they shrank Faerun to make travel times less onerous, so a party could reasonably travel back and forth, whereas with 5E they returned to Greenwood's absurdly huge continent.
Baldur's Gate and Waterdeep are over 1000 miles apart as the crow flies, may or may not take months depending on how you go, but from playing Hoard of the Dragon Queen I can confirm it is a long haul. And Baldur's Gate is not the southern end of the SCAG region, Eaterdeep is only about halfway up the coast, there are a ton of islands to the West and a huge area detailed to the East. It's a big territory.