The default display is in Scaled View. This plots all stats scaled to 100%. So you can compare the types of stats equally between all three campaigns, regardless of whether they were sent to the same amount of people.
But you can click on the % button at the top to view the stats displayed in Percent View. This view compares the percentages of all stats to each other. So a campaign with a 100% engagement rate would exceed other compared mailings, even if it was sent to the smallest list.
The No. button will display the Actual Number View. Here we're comparing the actual number of contacts that viewed, engaged, bounced, etc. So a campaign sent to the most people will overtake the display as the largest bar on the graph.