Well, they're right. It's pretty theoretical how that comes about but essentially it is because for a fluid flowing with a constant speed, this must occur:
Viscous ******ing force = Pressure differences at the two ends of the pipe
Setting that up gives us:
4πnLv = (p1-p2)A
When you solve for v you eventually get:
v= (p1-p2)r^2/ 4nL
Therefore doubling r will increase velocity by 4. There are other ways I can show you this but this is I believe, sufficient enough. You can always think of how Q (Flow Rate) is derived, and you'll also see some proof as to why. Let me know if this has helped clear your thoughts.
