I think its largely genetic: ie: somebody with a favourable posture and or neurocontrol has less trouble keeping balance with free squats, hence his/her waist does not have to struggle as hard to compensate being thrown offbalance, also what kind of fibers makeup your ab muscles etc. Also the length of the joints determines how much stress certain musclegroups receive during the lift.
If you feel your ass or waist becomes too thick from 4-6 reps heavy suats or deadlifts, just do your squats in the ultra high reprange : sets of 60 reps. Now legs are a special musle that will often respond well to megareps (just watch the turbowheels of some competetive 5-10 kilometre iceskaters). The funny part about is that your waist will become tight and well conditioned with these amount of reps, but not a thick crude blocky Yates waist. Changes are good that your ass will not balloon and look more like two tight coconuts.
as for the deads, for this reason, do them in the 20 reprange, also they are still superintens, mike mentzer recommended 20 reps squats/deads for its sheer brutallity.