This gets asked very often so I think it deserves a complete answer.
A coat which is too large CAN be cut down, and since it a big job, will cost a lot of money. Tailors don't frequently turn down jobs which will bring them lots of money, so few will tell you it can't be done. But is it possible, properly speaking
, to take a factory-made coat and turn it into a size 38? NO. But let me explain.
This is a view of what we call a "graded nest"- all the sizes of a particular pattern. From size to size (i.e. 38 to 40) the back circumference grade is Â¼", the side body is Â¼" and the front is Â½".
The jacket can be torn down and the back reduced at the side seam and the allowance at the neck let out in order to make a smaller neck, and the whole shoulder line lowered to make a smaller armhole. A lot of work, but can be done.
The circumference of the side body can be brought in at the side seam, but you are missing some fabric in the top of the armhole so the tailor will have to reduce a bit at the front, not the back. Small shape distortion, but nothing too bad. (NB- bespoke tailors leave an inlay here which can be used to correct this)
Now the front, where the change from size to size is biggest. The front panel develops at center front, so this is where the cut down should take place. Oops, there's a buttonhole, so you can't do it here, the adjustment will have to go to the side of that panel. Oops, there's a pocket there. Removing the pocket and resetting it is a BEAST to do and most alteration tailors will refuse to do it, if they even know how to. So let's assume you have a very skilled tailor who will open the pocket and reset it (which will cost you a fortune), removing the half inch at the side seam instead of the front. Unless the coat was cut in a bespoke shop you won't have enough outlet to shift the neck point so it will have to stay more or less where it is, which is now in the wrong place in relation to the center front- in tailor's speak, the shoulder will be crooked (search straightening and crookening the shoulder if you're interested) which can lead to a host of problems. But chances are, your tailor will not open the pocket and so may take the chest and waist down at this seam (which will give you a large-looking hip) and take the rest out at the back, in which case the balance of the coat has been thrown off.
The top sleeve can be reduced but the under sleeve can not (again, bespoke tailors leave an inlay here which can be used to correct this).
So while it CAN be cut down to be made smaller, the fit won't be exactly what it should be- you may get lucky, but the chances are greater that you won't, and that's an expensive gamble.