One reason given for varietal size difference of the pollen baskets on returning foragers is the stickiness of the pollen. Some flowering plants rely on insects such as our beloved bees or other animal vectors while others rely on wind to transfer pollen from flower to flower. Wind transfers small, non-sticky pollen best but plants relying on insect vectors may be more successful with production of sticky pollen. Of course, honey bee foragers can improve the stickiness of the pollen they pack onto their corbiculae by adding a little nectar to the pollen.
Some windblown pollens are collected by bees, perhaps due to their availability when other pollens are scarce. One windblown pollen that is used by bees is sweet corn pollen, often available during late summer times of lowered nectar flow.