Compare function in cartesion coordinate system and in polar coordinate system

Question

I plotted the function $$f(x, y) = 0.5*x^2+y^2$$ in Geogebra. Also i plotted this function but in polar coordinate system $Surface(u*\cos(v),0.5*u*\sin(v),u^2,v,0,2*π,u,0,3)$. Unfortunately, the plot of these two functions is different and I do not understand why plots of functions.