Re: how effective would the lighter heavyweights be today?
At the risk of stating the obvious, some of the smaller heavyweights would do better than others. The genuinely great ones would be better able to overcome size disparity, and stylistic considerations would also play a big part. The all time finishers would be the most sucessful.
Some fighters who proved in the ring that they would be effective the bigger modern heavyweights include:
Could they bulk up today?
What is less certain is whether it would make them better.