because you''re system can''t handle all of the polygons and/or the fill rate. the bigger it is the more polygons you''r drawing. on most 3d hardware that shouldn''t be a problem. try adding these lines to your code: glEnable(GL_CULL_FACE); glCullFace(GL_BACK); that way only the polygons facing you will be drawn.
Never underestimante the power of stupid people in large groups.