How do I excercise/make my chest bigger (male?

Why do bodybuilders today have proportionately bigger thighs and much more back depth than they did in the 1970s, while their chest and arm growth has not been as pronounced?

• This picture is illustrative. Note that  the circumference of Coleman's thighs is 28% bigger than that of Arnold's thighs, where as his arms are only 9% bigger in circumference and his chest/back is only 2% bigger in terms of circumference. If the length of a cylinder or a thigh is held constant, then a 28% increase in circumference translates to a 64% increase in volume. So it's probably more accurate to say his thighs are 64% bigger while his arms are only 19% bigger. Of course, Arnold is 6'2" while Coleman is 5'11" so Coleman is even bigger on a height adjusted basis. If you look at Jay Cutler, his thighs are 16% bigger than those of Arnold while his arms are actually the same size.[1] So his thighs would be 35% bigger those of Arnold in terms of volume. Jay is 5'9", so his thighs are even bigger on a height adjusted basis. It's like that for a lot of bodybuilders today. They all seem to have proportionately bigger thighs while not showing as much growth in other parts of their body. Here's a photo that illustrates that back depth difference. Arnold(right) vs. Coleman(middle) vs. Dorian Yates (left).

• Answer:

  Just a guess, but I would assume it is the combination of two things, change in the aesthetic judging (leading to more emphasis on training those areas), and the discovery of more effective training methods for those areas, bringing them more in line with the arm/chest training.