Make sure you're passing the AllowModeSwitch flag in your swap chain description. Without it, DXGI will simply use the desktop resolution.
Nevermind, I just noticed you're passing this to ResizeBuffers already, which is correct. However, it looks like you only set the client size of the window when in windowed mode. I'm pretty sure it's necessary that you also set the size of the window when in fullscreen mode. ResizeBuffers will only resize your backbuffer; it's probably get squashed back down to fit on the form surface.