I agree, a z axis or depth could be causing that. I just bought Design Cad 3000 Express by Upperspace because the box said it supported DXF export. Well, it does, in x/y axis only. Stretching the z axis in Bryce was ineffective. When you have the model highlighted, click on the "A" on button for attributes and look at the numbers on the bottom of the box. If one of the numbers is very low, like 0 or so, put in a larger number, around the same as the x and y. See if that help. If not, well, that's your problem. More info would help us help you.