
Great physique, no doubt about it. Even in Boogie Nights when Mark was 138-lb, he had clearly defined pectoral separation lines. That's obviously just genetic, based on the structure of his rip cage. In contrast, I only ever show pec separation like that when I am fairly lean [~15% body fat, which is about as lean as I get], and have a chest pump.
Otherwise, I just don't show that "shelf" look to my chest, that Getbig's own Tim Wescott has, for example.
Speaking of which, Tim is 5'8", and competes around 165-lb or so, doesn't he? He is likely more muscular than Mark Wahlberg is, but clearly he has great chest genetics in the same way that Mark does. Both have clearly visible chest separation, which is something I don't think I will ever have. Any chest separation I have comes from the overhang of my chest...not from natural pec lines like Mark Wahlberg has.
Mark can probably be 120-lb, and still have clearly defined pec muscles. A friend of mine has that too, and you can literally feel the indent where the pec line is. No matter how big - or small - his chest is, he will always have clearly defined, separated pecs.