My memory - such as it is ... back in the seventies, the tight jean was fashionable, and the manufacturers capitalized on this fashion trend.
Believe it or not, but what I recall was that the manufacturers of jeans actually cut the jeans differently from what they did before, or since. Namely, they knew that the majority of guys, being right-handed, tended to stuff there junk down the left pant leg. For years, manufacturers had provided extra material on the left side of the crotch to accommodate this. With the buying public wanting / desiring a very obvious and showy bulge, the manufacturers responded by removing material from the left side of the crotch of the jeans, so that the outline of the bulge became very, very obvious.
When tight jeans went out of fashion, the manufacturers reverted to the old way of cutting the jeans, and invented "designer" or "logo" jeans, to enable the jeans to become far more expensive. Levis / Wranglers were thirty bucks, and suddenly Ralph Lauren and Calvin Klein jeans were $150 - $300 bucks a pair. What a surprise.
Unless you go for Ruffskins, which are notoriously low cut, and tight tight tight in the crotch, you would not likely find a pair of jeans today that will emphasize the bulge. Unless of course, you buy a pair, then have a tailor or seamstress alter them for you. That is why the 'skinny' jeans don't show any bulge either.