For the women: Boobs

How broad or narrow one's back is at the equivalent of the under bust level contributes to the whole measurement. How much coverage one has over your back/ribcage can also make difference. Possibly see if you have anything left to pinch at that level, this might indicate if you have much if anything left to loose at that level. I know UK bra sizes are unbelievably awful.

I'm not sure any real/effective study of how one's stature effects clothes sizing needs has ever been completed. Look at the way children/girls clothing goes up to about a 28 chest then these children are thrust into "adult" clothing, with vat applied, if they happen to be larger at the age of 10 or so. There seems to be a fear of small women having access to clothing which does not carry adult taxes.

The average UK size used to be 14 for a young woman, the height assumed was 5'6", with anything going over and above but little if any availability under. So for those over 5'6" they had to wear them short, for those who only made it to 5', these clothes hung on them, shortening them still left the waistline skewed out of place. 10's for example were hardly seen when I was growing up. I've not made a recent study of the racks but think 8s are few and far between now.

Then there was the idea small chest sizes do not require support. I'm sure there has always been a void in this what I can only call the "in between" area. I know none of this helps. I'm really sorry.

Cups are very difficult to alter. Its fine for those who like extra space but having seen really soar skin related issues in the elder community from poorly fitting under garments (I once supported their personal care) having a good well fitting bra is essential even in the elderly. As a youngster (12) I had repurposed ones from my grandma with stitched beaks in them!! Mother thought them good enough, they were really uncomfortable. Breast tissue at any age is sensitive. My hand stitching has been really high quality but for achieving the good flat but curved, smooth seams, I would want, I'd need my swing needle machine to ensure un-abrasive coverage of the edges. Call me fussy if you wish. Women deserve better. As in all things, if men needed these things, had to wear them it would all be, very different. Earlier today on a program on marketing, I heard about Ann Summers uk, in the '80's their garments were designed by men!! Based on eye appeal not comfort, things take more than a life time to change.

You might have luck increasing your band size. Perhaps, work on your lats and upper back workouts?

I'm popping my head back in here to respond to the sister sizes chart that @MaltedTea shared. Yes, sister sizing is a thing, and yes, volume-wise a 42DD is comparable to a 26J, but those two bra-wearing humans with bodies are going to look very different and have very different proportions/distribution of breast tissue, so those bras are not going to fit the same. If you measure to a 26J, for one thing the band is going to hang off of you; even without stretching it, a bra with band size 42 is going to be probably 35-37 inches long, fully ten inches bigger around than you are. Maybe you can get creative with rixie clips or shorten the band if you're a decent seamstress, but then, the cups on a 42DD bra are going to be much wider and shallower than your breasts probably are - imagine putting an orange in a cereal bowl. Similarly, a person who measures to 42DD might be able to get their breasts into the cups on the 26J (though I expect those cups will be much narrower and deeper than the 42DD person's breasts are - more like an orange in a glass), but the band simply will not stretch far enough to get around their back; it's going to fall short by 10 or so inches (assuming a maximum stretched length of around 32 inches; many bras in that size range won't stretch even that far, though). Going more than maybe two steps in any direction on the sister size chart is going to result in a less-than-ideal fit, nevermind 8!

Now, none of that accounts for individual preferences wrt support needs, clothing tightness, budget space devoted to undergarments, or just how much damn work one is willing to put into finding a bra that fits properly, but if it's something you have the (mental/emotional, physical, financial) resources to do, it's very worth doing.