I have the same problem and I'm thinking it might be due to a too wide yoke in combination with a narrow waist. Hopefully someone more knowledgeable can
Could you please elaborate how a larger sleeve width would help? I'm thinking a wider sleeve would likely mean a larger arm hole and an even worse flying squirrel syndrome?
More length in underarm/sleeve seam. Translates to a shallower sleevehead and therefore a wider sleeve which doesn't look as clean in a shirt that is overfitted, as many are trying to achieve.
Problem seems to be shirtmakers retaining traditional shirt pattern forms but customers changing the fit parameters. There is a limit to how fitted a trad shirt pattern can be made, before it creates other issues.