Circle turning into an Ellipse with responsive design

Hi,

How can I prevent a round Circle from becoming and Ellipse when the project is scaled down?  At first, I thought this was due to "auto" settings in the properties panel, but I have set the values to be percentages, so I'm unsure as to why this is still happening.  The Circle starts to become an Ellipse when scaling down around the "Tablet" dimensions. 

Any help would be greatly appreciated. 

Thanks.