What specific FFT VI are you using? My guess is that you are using "Amplitude and Phase Spectrum.vi", which first computes the FFT using "FFT.vi" (which gives a full, two-sided spectrum as expected), then converts it to a one-sided spectrum. At any rate, I think you'll find that "FFT.vi" gives n output points for n input points. I've attahced a few screenshots: FP and BD for a simple VI that demonstrates that "FFT.vi" gives n outputs for n inputs and that "Amplitude and Phase Spectrum.vi" gives n/2 outputs for n inputs. Also in the screenshot image is the BD for "Amplitude and Phase Spectrum.vi" to show their approach for giving n/2 output samples.