Ks-band images of 20 barred galaxies show an increase in the peak amplitude of the normalized m=2 Fourier component with the R25-normalized radius at this peak. This implies that longer bars have higher m=2 amplitudes. The long bars also correlate with an increased density in the central parts of the disks, as measured by the luminosity inside 0.25R25 divided by the cube of this radius in kpc. Because denser galaxies evolve faster, these correlations suggest that bars grow in length and amplitude over a Hubble time, with the fastest evolution occurring in the densest galaxies. All but three of the sample have early-type flat bars; there is no clear correlation between the correlated quantities and the Hubble type.