Didn't see any other threads addressing this, so thought I might share my knowledge!
There's 4 new ini files that all effect various training areas. The main ones I've looked at are training_drill_attributes_list.ini, training_progress.ini and training_result_converter.ini...
Actually, just looking at mine again, this wasn't required. Physical traits grew as long as the max attribute growth per season was set to a decent number, max monthly attribute growth was set to a decent number, and all the curves had the same values (I.e., 100).
I seem to remember a similar issue with FIFA 14. From memory, you need to change the phys_x value to a lower number. For instance, the first one is 16 I think. Try lowering that.
Also, I think changing the form_age_division helped too, but my memory is a bit sketchy!
Fairly certain there's some differences between the FIFA 14 and FIFA 15 playergrowth.ini. Just extracting the 14 ini and putting in the same spot for 15, results in players not growing at all (that's with no changes whatsoever to the ini file).
Did you use the FIFA 15 ini? If so, how? I just...