Keynote 6.6: How can I make an object wider by animation, but not higher
One way to accomplish this is with the Magic Move animation.  However this will require an additional slide.  Duplicating the slides including their content will not give the appearance of a slide change, only items that are moved or changed will appear to animate, not the slide itself.  See animation below.

Create your object on one slide.
Duplicate the slide.
Resize the object in the second slide to the desired width.
Set animation on the first slide to Magic Move
Change the animation duration and/or acceleration to the desired result.

Slide one:

Slide two:

Resulting animation: