How to avoid automatic resizing of axes

I create a simple graphic interface using uicontrol & uipanel that contains 2 axes. My problem is that, when using camva function, the axes size does not keep constant. First axis can overlap second axis.

See a simple example:

function example

fig = figure;
set(fig,'position',[25   75   600   600]);

hp = uipanel('FontSize',12,...
    'BackgroundColor','white'...
    );

a2 = subplot(1,2,2,'Parent',hp);
plot(a2,rand(1,10),'r');

a1 = subplot(1,2,1,'Parent',hp);

npanels = 72;
erad    = 6371008.7714;
[x, y, z] = ellipsoid(0, 0, 0, erad, erad, erad, npanels);
surf(a1,x, y, -z, 'FaceColor', 'b', 'EdgeColor', 0.5*[1 1 1]);

uicontrol('Parent',fig, 'Style','slider', 'Value',2, 'Min',1,...
    'Max',10, 'SliderStep',[1 10]./360, ...
    'Position',[150 5 300 20], 'Callback',{@slider_callback,a1})

end

function slider_callback(hObj, eventdata,a1)

            angle = round(get(hObj,'Value'))
            set(a1,'CameraViewAngle',angle);
            get(a1,'Position')
  end

When using slider callback for the 1st subplot, the sphere can overlap the second subplot. How can I avoid this and enforce that the axes position and size remain the same?

Thanks in adavance, Petrus

Product version: Matlab2007a