Go to the following link.
http://bulletin.ipm.illinois.edu/pastpest/articles/200323h.html

 Here is the section that might answer your question.
 

Soybean test weight doesn't seem to vary that much, and it doesn't get much attention. Soybean seeds tend to be more regular in shape than corn seeds, and spherical shapes like soybean seed tend to fit together more uniformly regardless of size differences. A shortened seed-filling period may also result in smaller seed but not much change in density of individual seeds