Misbehavior in PlotType.Cartesian

Hello again,

I have observe a misbehavior when I set
PlotType = PlotType.Cartesian

When my application re-sizes reducing it's size, the plot also re size with proper proportion.
But when the application re-sizes increasing it's size, the plot re size itself but keeps proportion as it is.

Kindly let me know how can I avoid this issue.

Thanks.