Question

For comparison with other years and with other countries, we prefer a histogram of the percents in each weight class rather than the counts. explain why.

Answer 1

It's because Different years and countries may have different overall numbers of newborns, making a comparison based on the absolute numbers difficult 
If we insist on using the absolute numbers, The group with the most population would definitely result in higher results, and make it impossible for us to see the changes in weight that happen to average inborns.