There are commonly three categories of women who have always carried larger bags (although that may have changed with the fashion trend of large bags): moms, older women and professionals (carrying files or computers, etc.). At least two of those categories also tend to be generous (moms have to be and many older women never let go of the mom mindset that everything they do or buy must be for someone else).
Again, stereotypically speaking, younger women tend to carry smaller bags. They also tend to be more selfish - not in a bad way, but just because youth often makes people myopic in their viewpoint. Because they have fewer familial obligations, they are allowed to put themselves first.
Again, these are just stereotypes and by no means hold true for everyone. There are plenty of moms and older women who are selfish and plenty of young women who are very giving. But on average, I think that would explain the findings.