Sure, the pie chart would indeed tell you which slices are big and which are small.

It pays to measure numerically though, rather than describing the slices as merely big or small. The question of how big a slice should be described as big enough for attention becomes subjective. You will have to do a quick mental arithmetic to arrive at the cumulative 80% of the slices.

Once I have the cumulative frequency curve in a Pareto Chart, the method I commonly use is to project the 80% mark from the Y axis to the curve, and drop the intrecept down on the x axis from that point. All bars to the left of this dividing line can be treated as important, since they will be addressing 80% of the effects. Once this is done, I turn my attention to another process where another 80% of pressing matters demand attention.

I find this a very effective way to prioritize.

Let's say the Pareto Chart makes the 80-20 law more 'visible'.