It's all about mass to grow like you said. if I have one square inch frag and it grows 1/4 for every inch it has of side it will grow 1 hole inch in square space in one month. Now if I have a 5 inch frag and the same growth 1/4 inch for every 1 inch real-estate it has on it's side. This frag will grow four square inches each month. Like stated above bigger frags/colonies grow faster. Feeding it and it having more eyes does also help.
Disclaimer- Yes some coral does grow faster then others but more mas to grow from does make it grow faster, this also is shown with my hanging coral makes it grow faster then putting it on a frag disk.