Barbour jackets can be tailored. It won't be cheap and 99% of your local tailors won't touch the work (from my NYC experiences).
You have to go to either someone who has worked on Barbour Jackets before and KNOW WHAT THEY ARE DOING.
Or you can take the chance with someone who has never seen a Barbour coat before and are willing to try because they "don't see why not".
Both situations will likely cost you anywhere between $50 and $100+ in work.
I won't recommend that you try to budget this since a crappy $50 job will cost you a $379/$399 jacket if done poorly. So go to the best tailor you can find with in your budget. Also one that will not try to rip you off since having a Barbour coat may mean to them you are a walking pay day
(recalls to mind my first BMW oil change experience years ago when the oil change guy wanted $200 more than my girl friend's Honda Accord because the BMW was an Imported Car. I refused saying the Accord was also imported and my car being a BMW used the same laws of physics to operate. It wasn't some magical unicorn that required extraterrestrial attention.)
Like the Beaufort the Bedale Jackets can be tailored or rather would be easier to tailor. They have riding vent flaps and armpit vent holes adjacent to the flank seams. So you can't really tackle a width issue there unless you want to delete those features (or have them oddly shifted about).
I have seen a few used tailored Bedale Jackets on eBay and auctions where the center spine (collar to hem) had a new seam reflecting that the jacket was taken in there. This looked quite good BUT the side effect was in interior labels were deleted for the tailoring. As shown below the Bedale's interior labels are in the center.
The other type of tailoring was two hem lines (also collar to hem) opposite the spine about 10 inches apart. This also looked good and the interior lining labels were spared BUT the riding vent flaps were pulled in towards the center and looked awfully narrow and odd.
I think the creation of a new center back hem on a Bedale and Beaufort can be done to look decent (you most likely can't take more than 3 inches off before it looks odd) but you need to make sure the tailor respects the location of the inside center brand labels and don't toss them out or puts them back in place when done with the work. Also for the Beaufort the game pocket will have to be properly tackled.
The sleeves can't be made longer but you can shorten them (this I've seen done many times).
Also keep in mind that Barbour jackets tend to stretch out after the first few weeks of wear. The raglan sleeves tend to stretch out an inch or more with wear.
I was also told "no". The issue concerned the lack of a central seam in the back of my Beaufort; material would have to be removed from the sides, where there is a lot going on (game pockets in rear, close to bellows pockets in front.
From 2009 to Winter 2014 it pains me to Barbour now makes 100 contemporary jackets and products for every one Bedale or Beaufort or Border jacket they make a season. Most of this contemporary jackets are factory made in E.Europe, India, Asia. As a result the new jacket lack the original hand made tailored quality.
But yeah you can find slimmer cuts in something like a Bedale SL or a Dept. B jacket. These will cost you more money than the originals though as they can easily cost above $500.
Have you tried their contemporary line / slim-cut jackets? Every season they add new slim models or adapt the old ones.
e.g the
Contemporary Beaufort
The Barbour wax never stained my clothes and I've worn 'out' dozens of Bedales and Beauforts since 1998. In fact the wax finish has greatly improved in the last years with the Sylkoil process that they use which makes the wax finish more durable for daily wear requiring less treatment and maintenance than the older wax coats.
I don't understand these "oil finish" coats. Filson says, "Entire coat lined with 4 oz. cotton to prevent Oil Finish Cover Cloth from staining your clothes." Okay...so the manufacturer states that the finish will stain other clothing...which in turn means you can't safely store these things next to other garments. You also can't throw them over your shoulder, and you can't sit down in them and lean back against any sort of fabric that you wouldn't want to stain. Seems like that all adds up to this Filson coat being an outdoorsy thing only. Right? And the coat should probably be stored in its own hanging canvas bag.