I believe "deflection" is the result of "flex".
What you say about smaller tips and regulation size balls seems to make sense, but then 3-cushion billiard players play with even larger billiard balls and they also tend to use thinner shafts...
No they do not, while the tip may be 11.50mm the shaft itself is close to 13.00mm about an two Inches from the ferrule and it only gets larger from there.
I have played 3-Cushion for many years and I have never seen the smaller shafts you are describing.