From large to x-large

[Frodo]

I am making vests with a pattern that only goes up to large

I need to make an x lg and a xxlg using the lg pattern

the pattern is 4 pcs

2-front, 1-back yoke. And the back panel

if I increase the width of the back panel 1” on both sides (2” total). Will this be sufficient? 

[Aven]

Frodo, you can add width, but the armholes might  be uncomfortable.  If you can stack the cut out patterns large, medium and small, you will see how the armhole shape changes from one size to the next.  You might be able to figure out an XL and an XXL pattern from just this.

But if this was me, I'd look for a denim jacket from the thrift store as a guide. A sweatshirt should work as well and will probably be cheaper.  You don't have to cut them to use them, just pull the arm into the body. 

If you know someone that is the right size you could ask borrow a piece of outerwear use to trace a pattern.

This might help. https://leatherworker.net/forum/topic/2463-biker-leather-patterns/

This is a painters vest pattern.  Its digital so you will have to get it printed, but if they stack all the sizes on one pieces, you'll see how the armholes change.  Its only $9, so cheaper than the thrift store jacket. Painters Vest