Generally, single breasted trench coats are more flattering than double breasted ones. This is particularly true if you are well endowed or carry a little extra around the middle. But even slim and well proportioned women sometimes don’t look right in double breasted styles. So I’ve been puzzled in the past when I’ve dressed curvier women in double breasted styles that have worked really well.
The secret lies in the relationship between the height of the person and the horizontal distance between the double row of buttons.
Buttons that are widely spaced on double breasted items tend to have “a widening effect”. Taller women can carry off these styles because the larger spaces are proportional to their height. On the other hand you’ll automatically get “a slimming effect” once those buttons are pulled in closer together. In the extreme a single row of buttons is still the most flattering option for shorter and curvier women.