Question
My Corn Plant is 5 ft tall.  But the stems are thin.  What can I do to make them grow thicker.

Answer
Please refer to the following page on Dracaenas posted by the  Clemson University Cooperative Extension Service:

http://hgic.clemson.edu/factsheets/HGIC1504.htm

Some cultivars of your Corn Plant have naturally slender stalks.  Dracaena marginata has 'Thin stems are topped by clusters of slender arching leaves.'  Dracaena godseffiana has 'thin-wiry stems' and leaves with creamy speckles.  Under the best of care, these Corn Plants will never have thick, solid trunks.

Bright light tends to make all plants stockier.  To give you an example of how light can affect a plant's height, let me point out that summer-blooming Lilies grow tallest when they are not growing in Full Sun.  And to lengthen the stem on a Tulip in the spring, they can be covered to block all light.  But these stems are not strong and the plants are less healthy because of the light shortage.

To get your Corn Plant to grow the thickest stem it is genetically capable of, you must give it the most light it can take without burning the leaves.  This is not a full sun plant and that will take some work.  Slowly increase the light and watch carefully to make sure the leaves are not turning brown in the middle -- stage one sunburn -- due to too fast increase in sunlight.  Good luck and keep me posted.