Why is corner radius changing when 9-slicing?

[Skylexion (Sky)]

I’m trying to 9-slice an image so it’ll fit propertly and keep the same resolution for all devices. I’m able to move it whatever size and the shape stays the same but whenever I got to a smaller device (i.e. iPhone 4S) the corners get super round. Why is this happening and how do I fix it?

What I want:

What I get:

Thanks,
Skylexion