From the BLS PDF: >Full-time workers continued to be much less likely to be among the working poor than were part- time workers. Among people in the labor force for 27 weeks or more, 4.1 percent of those usually employed full time were classified as working poor, compared with 15.8 percent of part-time workers.

That doesn't imply choice. Their clearly can and is a positive correlation between working part time and being unable to find full time work.