This is true also for "English" horses. The elastic on girths allow people to tighten their girths so tight. And Some of the synthetic ones have a bump right where this vein is causing even more distress.
Hollow vs round frame horse diagram. (Now you finally get what your instructor is telling you)
read this little article about the position of the saddle. it is really important that it is put on properly!
Don't place the saddle too far forward, on top of the shoulderblade
saddle fit More
These aren't the only things out there of course, but it should get you started and keep your drawing busy. Available in smaller, more digestible chunks at www.htdhorses.com :