If you give some people a little or yield anything, they will want more and more; some people are never satisfied. I gave Billy a bite of candy and he wanted more and more. If you give him an inch, he'll take a mile. The counselor said to Jack, "No, I can't let you get a haircut until Saturday. It's against the rules, and if I give an inch, someone will take a mile."
GIVE ONE AN INCH, AND HE WILL TAKE A MILE
Meaning of GIVE ONE AN INCH, AND HE WILL TAKE A MILE in English
American idioms English vocabulary. Английский словарь американских идиом. 2012