It's not really about the heli size but more about the battery (they are linked of course). The integrated bec used on an esc which is connected to a 11.1 volt battery doesn't have to work that hard in order to get down to 4.8v which is what your receiver and servos want (mostly, some will work on 6v but lets assume it's 4.8v). When you move up to a heli larger than 500 size though you tend to start using 22.2v batteries. This means the bec must reduce the voltage from 22.2v to 4.8v or 6v. This is when you want a separate bec, the amount of voltage drop can create a lot of heat and stress which can not only affect your esc, but also make the bec fail entirely, this would fry your receiver and servos.