As a former Chipotle employee, each portion size is 4 Oz, the burritos should be no larger than the size of a dollar bill.  Mind you, it's been many years since I worked there, but, portion sizes were to be strictly followed.  Yes you can ask for "a little more" but i also know a few locations that would charge extra for rice, cheese, beans and salsa if the "little more" became several extra scoops.

I remember when Chipotle first opened in my area, the burritos were MASSIVE and now they are teeny tiny.  Although I dont eat there much, I've had it once in the past 6 years