The Preferences choice "Smooth voxel-based images" has an impact on these images. The issue is that the voxels in a volume do not have a one-to-one correspondence with the vertices in a mesh. In general, if your data is pre-thresholded (artificially surrounded by zeros) you must either use nearest-neighbor interpolation (which makes surviving regions look smaller and with jagged edges) or trilinear smoothing that can blur the edges. Personally, I prefer using unthresholded images so the non-significant voxels are able to accurately influence their surviving neighbors allowing a better interpolation. The MediaWiki describes this in more detail.