Problems Resizing Project

I am trying to resize a file (see attached) from a custom size of 960x540 pixels to 1200x675 (I have locked the aspect ratio). When I choose Scale to Fit, all of the elements of the slides scale beautifully except the font. The font sizes increase so dramatically that the text begins to spill off the slide. Am I doing something wrong? 

 

For example, this slide looks like this before I scale it:

 

After I scale it, it looks like this:

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