"To increase the spread" means to widen the gap or distance between two things, whether it is the distance between two points, the difference between two values, or the range between two options. Full definition
In summary, credit expansion increases the spreads between revenue and costs, increasing profit margins. (mises.org)
The high level of bad loans worked to reduce the overall spread, while banks sought to compensate by increasing the spread on performing loans. (rba.gov.au)
Unfortunately, staying indoors inevitably increases the spread of germs. (mindbodygreen.com)