Symptoms

After upgrading to 4.6.9, a form with a Text Area control loaded directly on the form level, causes rendering (form display width) problems.
 

Diagnoses

This is related to the Text Area control itself. If you take the Text Area control off the form and replace it with a Text Box, the decreased form width is no longer occurring at Runtime.
 

Resolution

This behavior was logged as a bug to be fixed in 4.6.10:

Bug 536438: K2SF, Textarea control causing rendering problems, no watermark.

Adding a watermark value to the Text area control causing the problem seems to remedy the behavior.