i disagree, 75kg at 5'10 is fairly average. 70 would be fairly thin. if your doing push downs with most of a stack but struggling with dips it would make me believe thats a well small stack.
at goodlife i'll do push downs with a little over half the cable stack at the end of an hour long chest/tri/shoulder session and still be able to do 3X10-15 reps of body weight dips. i'm 78kg and 175cm tall, so i'm hardly what you would call huge....