Certain styles are now arriving from Italy with a smaller sizing if you sometimes buy medium or large we would advise simply buying large as below the difference is around 1 inch on UK fitting.
Small = UK XS/Small 37-38" chest (20-21" pit to pit measure).
Medium = UK Small/Medium 38-39" Chest (21-22").
Large = UK Medium/Large 40" Chest (22").
X-Large = UK Large/XL 42" Chest (23").
2XL = UK XL/2XL 44" Chest (24").
3XL = UK 2XL/3XL 46" Chest (25").
4XL = UK 3XL/4XL 48" chest (26").