Try something like this. S is a Affine transformation matrix for scaling.
% circle of radius 1 for demo
th = linspace(0,2*pi) ;
x = cos(th) ;
y = sin(th) ;
o = ones(size(x)) ;
C = [x ; y ;o]' ;
% up scaling by 1.5
cx = 1.5 ; cy = 1.5 ;
S = [cx 0 0 ; 0 cy 0 ; 0 0 1] ;
C0 = C*S ;
x0 = C0(:,1) ; y0 = C0(:,2) ;
% downscaling by 0.5
cx = 0.5 ; cy = 0.5 ;
S = [cx 0 0 ; 0 cy 0 ; 0 0 1] ;
C1 = C*S ;
x1 = C1(:,1) ; y1 = C1(:,2) ;
figure
hold on
plot(x,y,'k')
plot(x0,y0,'b')
plot(x1,y1,'r')
