How did bodybuilders in the 70' have a small waist?

That's because they weren't using HGH back then. HGH helps put on a lot of muscle hence the body builders are a lot bigger than what you used to see before 80's, but the major side effect is a really big gut because of insulin!!

Back then that was the aesthetic people liked and the aesthetic that made them look better today it is not like that so top bodybuilders have different aesthetics .

Not all of them had a natural V taper so they found another way by making their delts bigger, lats and back wider.

There are a variety of answers, some of which have been covered by other authors, but a significant portion of the credit goes to a (presumed) combination of chemicals, diet, and muscular development.

The bodybuilders of yesteryear at a lot of food, but not nearly the gargantuan quantities that the modern monsters chow down on. The body adapts to stressors: constantly fill the gut to the point of bursting, the gut will adapt.

In addition, the transverse abdominis — quite literally a girdle of muscle that wraps around your midsection — was more well-developed in the older bodybuilders. They would specifically target it with variations on the “vacuum”, and work on contractions that kept the midsection smaller.

They didn't do many weighted ab exercises. They would do extensive repetitions and sets of hanging leg raises, decline/roman chair crunches, etc., using their body weight, and otherwise rely on the heavy weight of their compound movements to strengthen their core.

Loading up the ab curl machine to the bottom plate and doing crunches will certainly strengthen your abs, but it will also thicken your midsection through the resulting hypertrophy (mass increase of the muscles). Additionally, any oblique movement (e.g. dumbbell oblique side raises) should not be done for very heavy weight or you will ultimately look as though you have love-handles, even when you're very lean.

Finally, many bodybuilders today who are not "natural" utilize growth hormone as a supplement and this causes an unattractive distension of the belly.

see you soon !!