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I saw that you are using ddSlick dropdown widget and seems that this problem is only happening in Chrome, not in Firefox. Widget is loaded dynamically through Java Script and seems that in Chrome, code that is making this widget getting additional width. Because this problem can not be reproduced on blank form I think that this widget is in collision with some other code on your form. For now only workaround can be to use other type of field like regular dropdown field.
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