Everything from environment to lighting to nutrients ect govern the growth of your plants! Even clones cut at the same time from the same mother grown under different conditions will grow out differently. That being said if he didnt top his then he would get alot more vertical growth due to apex dominance. When u top the plant u break that dominance and get alot more side branching going on. Was yours bushier than his? Was his grown outdoors or under stronger grow lights. U could have stunted her topping it but I believe theres more to it than that. Height at which u switch to flower also plays a huge role in the final height. Were yours alot smaller when thrown into flower?