In this paper, the spin dynamics and broadening mechanism of cesium vapor in cells without buffer gas is investigated by means of spin noise spectroscopy. In a macro atomic vapor cell, the lineshape of detuning frequency spectrum of spin relaxation rate is of Gaussian distribution. For a micron-scaled vapor cell with strong spatial locality, the lineshape of detuning frequency spectrum of spin relaxation rate is of Lorentzian distribution. The parameter dependence of detuning frequency spectrum of spin relaxation rate, such as temperature, is studied quantitatively. The detuning frequency spectrum of the spin relaxation rate is measured experimentally to be broadened by $ \sim $4 GHz, which is obviously larger than the unhomogeneous Doppler broadening of $ \sim $500 MHz for a macro atomic vapor cell. At the same time, the detuning frequency spectrum of total noise in the two atomic vapor cells is studied. In the macro atomic vapor cell, the total noise intensity strongly relies on the detuning frequency of the laser with respect to the atomic resonance transition. In the micron-scaled vapor cell, due to the strong homogeneous broadening, the center of the detuning frequency spectrum of the total noise is observed to dip. Finally, a simplified physical model is established to compute the broadening of the micron-scaled vapor cell. The homogeneous broadening of atoms is explained experimentally and theoretically in the micron-scaled vapor cell.