Why is level of power spectrum dependent on FFT resolution?
The observed phenomenon is simply due to the fact that when you set window length to 100 it is as if you were doing fft with 100 samples instead of 1000. So Nw=100 similar to NFFT = 100.

And why does FFT size affect results (level of noise VS level of tone)? Good question

Two types of spectral densities:

1) PWELCH-like PSD: "PSDW"


A tone grows in dB as FFT size increases. As in a true PSD the tone is a Dirac's delta for NFFT->inf. Larger NFFT means smaller dF, and larger A, in such a way that A * dF = constant
A unit power noise stays at 0dB. 


This is really a [Watt/Hz] spectral density and total power given by:

totalPower = trapz(F,PSDW)  =  trapz(fs * F,PSDW/fs)  = sum(PSDW)/NFFT

2) OSCILLOSCOPE-like PSD: "PSDO", PSDO = PSDW/NFFT 


A tone remains the same dB as FFT size increases
A unit power noise becomes less dBs as NFFT increases.


This is a [Watt/Hz/NFFT] measure, with power:

totalPower = NFFT * trapz(F,PSDO) =  NFFT*trapz(fs * F,PSDO/fs)  = sum(PSDO)