For those of us who are amply endowed or as a friend used to say "possess substantial commodities" we tend to outgrow jackets across the bustline. It still fits across the shoulders and the arms are ok.
Such was the case with my very, very favorite velvet jacket which is now pushing 20. Nipped-in waist, bustle in the back - what's not to love. And I almost consigned it to the donate pile.
I tried it on and it still fit across the shoulders and in the arms, but I was looking at a 4-5" gap in the front. My first thought was to take off the original buttons, sew identical buttons on both sides and run some lacing or chains. That might be cute with a corset underneath the jacket. But my eyes fell upon a couple of very old and cheap FOH (Fredericks of Hollywood) back lace corsets also on the way out.
This was the end result.
Now, what would I have done if the jacket was too tight or the sleeves didn't fit? Easy - if you are not afraid of a little ripping.
First, if the jacket has shoulder pads, take them out. This can be easy - just clipping a few threads, or major surgery if the pads were inserted between the lining and the outer fabric.
If that is the case, I usually open the lining at the shoulder seam very carefully, starting about a half-inch in from either the neck or the shoulder. In older jackets, the pad may be stiched down tightly.
If it's still too tight, take a close look at the lining. It may be the problem. Open the lining seam(s) starting about one-half inch from the neck, waist, shoulder, armhole, etc.
If the jacket is still a bit tight, consider adding an insert into the back or sides.
If the arms are too tight, but fit at the shoulders, consider opening the top or bottom seam on the sleeve but leaving it closed at the wrist. You could add a couple of pretty buttons on either side with ribbon ties.
Also, consider taking off the sleeves completely to make a vest. If you want the look of tie-on sleeves, sew some buttons at the shoulder and the top of the sleeves and use ribbons or chains to tie on the sleeves. Get fancy and make a shoulder roll or epaulettes.