I know what you mean!!!!!!
I have small shoulders, 14 inches across, but had the bigger boobs, so I have to buy larger bra's to fit.
I used my seam ripper and took out the stitches and move the straps over 1/2 inch to the center. This is the only way to keep them from falling off. I also do the same for my daughter's bra's.
I was using double face tape on the straps for awhile.
Syl