I think theres quite a few different reasons it appears more so throughout the ages specially now with all weights under the spotlight as way back at the start of boxing being filmed.

You find each end of the scale of what you are talking about in the different weights to an extent as well just naturally.

Because someone is a heavyweight doesnt necessarily mean he has a tougher beard than a flyweight it is only whats generated behind the blows so thats the way to get the job done faster at those weights.

The heavies were the most popular of weights to be broadcast to the public years and years ago so it leaves that impression even more.

You look into the lesser weights in each era though and you still find some great combination style fighters like Pep and his great opponents.

You're right, these days volume is something rewarded.(sometimes even though its not right on the dial) as Trueno said, which is a real shame.