Influence bubbles show how correlated, or how important, a driver is to your organization's engagement score.
A big, filled in Influence bubble indicates that the driver is highly correlated with engagement at your organization, and changes for this driver will make a big difference on your Emplify score.
Small Influence bubbles indicate that the driver is not as well correlated with engagement at your organization, and changes for this driver would not make a significant difference for your Emplify score.
How the score is calculated:
The influence bubble is a representation of the Pearson's correlation coefficient between the individual driver and the Emplify Engagement score for the organization within a single assessment.
In Real Life:
If your organization mostly employs at-home caregivers, questions on the Emplify survey about Friendship might not be as applicable to engagement for your employees. They knew when they signed up for the job they were committing to working alone and never expected to be friends with their co-workers. In this case, Friendship isn't paramount (or highly correlated) to their engagement over all, so their Influence bubble is small, even though their score is low.