I already reported this to Adobe. I'm just adding reference to this forum because this will probably help others.
With the release of CC 2015.1, in Mac only, the dropdown menu width will auto expand past the set width to the width of largest menu item. Prior to CC 2015.1, the dropdown menu would stay at the defined width. The dropdown items would expand when clicking the dropdown menu. However, the dropdown menu itself would not expand.
To test whether it was my scripts only, I added action sets and actions with long names and then opened Image Processor. This issue can be seen on the Image Processor Script too. The action sets with a long name will expand the action set dropdown menu width. However, after selecting an action set, when the action dropdown menu populates, it will not expand the width of the dropdown menu. Therefore, it appears that the menu items will not affect the width of the dropdown after the UI window is created.
As a work around, I put the menu items I needed for my scripts into an array. Then, I looped through the array during the window.onShow function. This fixes the issue for my scripts. I also tried putting the code to populate the dropdown menu right before window.show(). However, that did not work. The only way to make it work was inside of the onShow function.
Windows does not seem to have this issue. Also, I only have Mac Yosemite OS to test and it has not been tested on El Capitan OS.