Yup, DEFINITELY think less food = smaller piles! Definitely don't feel the feces would remain inside until the usual size pile was "ready"...& I think the smaller piles are a good sign since she's taking in less because it means things are moving through her in the normal/usual amount of time for her!
Good luck with the tests! I agree, they should be willing to simply do that which you as her mom feel is required, nothing more, nothing less...they may still "recommend" other diagnostics, but you do what you are comfortable with to ease YOUR MIND! Usually, extras are recommended to prevent "litigation", as in, so they can show they were thorough beyond compare. But always necessary, and as a horsewoman, thou know what you feel she NEEDS, given her age as well!
Keeping up the positive! Looking REALLY GOOD FOR HER I BELIEVE!! :0)