if I got it right, the AAL masks were smoothed. This smoothing will introduce non-binary voxel intensities, and SPM treats all voxels with intensity unequal to zero as in-mask. So, even small FWHM will lead to much larger regions covered by the mask than AAL. If your initial masks have intensities of 0 and 1, you should threshold them at something below 0.5 to get a dilation effect. Values between .5 and 1 would result in mask erosion.
Hope this helps,