Quoting tailor JefferyD from Styleforum
This one keeps coming up and most people are mistaken about the causes so here's a detailed look at what causes shoulder divots or dents.
First, the divots have nothing to do with the width of the shoulder- we often hear people making comments about a shoulder being too wide because it is denting but this is not the cause. Look at old photos of Tommy Nutter's work- you can't get much wider than that and they don't dent.
Second, the divots have nothing to do with the amount of shoulder padding; again, you can tons of it and not have dents, and you can have dents on an unpadded shoulder.
THIS is what causes the divots.
The armhole must be cut in the right shape for the body of the wearer. The sleeve is then cut in a very precise relationship to the armhole. In the figure below, the height of the armhole dictates the height of the sleeve cap and the width of the armhole dictates the width of the sleeve cap. We'll say that distance a-b must be equal to e-f and distance c-d must be equal to g-h (for the super geeks, this is not the actual formula but we'll say it is for simplicity).
If you put on a jacket whose armhole has not been cut wide enough for you, or that the chest pulls because it is too tight (or a host of other reasons the armhole may distort) the armhole will contract- it will get wider and shorter. The sleeve cap is now too long and narrow (a-b is shorter than e-f and c-d is wider than g-h) so it pulls from front to back, and the extra length collapses. THIS is what causes the divot.
The only way to try to remedy this is to remove the sleeve and shorten the cap (cut away excess length); this will, in some cases, be sufficient, but in many cases you also need some extra width to the sleeve cap, which you will not be able to gain since there is no outlet for it. This is neither easy nor cheap so your average dry-cleaner alterations tailor may not be able to do it.
The only way to know if a jacket is gong to do this is to try it on. If it dents, try a size up or try a different maker.