It's normally the case that vintage garments were sized smaller than their modern equivalents. We often find that jackets and suits from the 1940s through the 1970s were labeled with the size they actually measured rather than the chest size they were meant to fit. In those days, if you measured a 40" chest you bought a jacket labeled 42" - because a jacket labeled 40" would have been skin tight!
The one exception to the above are overcoats. Vintage overcoats were often sized larger than their modern counterparts. This is because they would usually have been worn over a suit or tweed jacket, and often over a vest, too. Add to this that vintage suits were made from heavier-weight cloths and you can soon understand why one would need extra room!
Modern coats, on the other hand, are frequently worn just over a shirt, while modern suit jackets are often lightweight and less structured than vintage garments. A modern coat might be cut with just 3 or 4" of spare room in the chest, while a vintage overcoat would be cut with up to double that amount.
HOW TO ENSURE YOU GET THE RIGHT FIT
1. Know your actual chest size. With your arms at your sides, measure around the fullest part of your chest, across shoulder blades and underarms.
2. Halve this measurement to get the underarm to underarm measurement.
3. For a fitted cut, or if you're not going to wear the coat over a suit jacket, look for coats with an underarm measurement that is 2" greater than your actual underarm to underarm measurement.
4. For a more relaxed fit, and for wearing over a suit jacket, look for coats with an underarm measurement that is 3-4" greater than your actual underarm to underarm measurement.
Example - If your chest measures 40" inches. Halve this measurement to get an underarm to underarm measurement of 20". For a fitted cut, look for coats that measure 22" underarm to underarm. To wear over a suit jacket, or for a more relaxed fit, look for coats that measure 23-24" underarm to underarm.