regarding leg size
Personally, i know it's a matter of leg size and bodyfat% to get seperation clearer... you have to also take into consideration the shape of the different heads of the quadriceps. I have a different shape pattern in my right side of the body than the left. my right quad cannot show any kind of seperation even with low body fat, because the heads are... how can i say this... intertwined i guess? ... i don't know the correct anatomical naming of the different heads, but the outter head overlaps and covers the medial head, and the tear drop goes up under the medial head. with the left quad, it looks normal and doesn't have any of those problems... so, again with genetics, it depends on where your muscle insertion points are and the shape they take.
regarding the stairstepper and stepmill to increase leg size... i have noticed that even when i'm in a caloric deficit, overtraining, doing cardio 2 times per day... etc. if i chose a cardio routine that involves resistance on the quads, hams, and glutes (i.e. high incline treadmill) my legs get smaller and feel burnt out... BUT, on my days off, i can actually see them growing bigger than before i started dieting! seperation increases, and the veins are just crazy! 2-3 days back to cardio and they shrink.