How can I delete surface and contour data outside of the zoomed axes?

I have an figure containing a number of very large surfaces (essentially a pcolor plot) overlaid with a contour plot of a different data set. Now I want to select a small area to reuse in another figure. I can copy it using copyobj(allchild(gca),newaxeshandle) and zoom using xlim([x1,x2]), ylim([y1,y2]), but that method carries along all the (many MB of) data outside the x,y limits I need. I'm not planning on zooming out again. How can I throw away the excess data?

Example:

%Draw a contour plot and check how big the data is:

contour(peaks),

ax1=gca;

ch=get(ax1,'children');

get(ch)

size(get(ch,'Contourmatrix')) %specifically

xlim([15,35]),ylim([35,45]);

%open a fresh axes as part of a new figure

figure

ax2=subplot(2,1,1);

copyobj(allchild(ax1),ax2);

ch=get(ax2,'children');

size(get(ch,'Contourmatrix'))