Xnest switches to full screen when I start startx in it

Question

I launch Xnest with

Xnest -ac :1  -geometry 800x600

then I can run xclock in it from another terminal with

DISPLAY=:1 xclock

but when I attempt to start X session in it from another terminal with

DISPLAY=:1 startx

Xnest window switches to full screen but does nothing except blinking cursor. I do not want it to switch to full screen.

Would anybody tell me how to do that?