It doesn't actually become thinner, it's only an illusion since the subpixel overlap between the border and body fades out of existence too quickly with VSFilter. The reason for the discrepency between VSFilter and Libass in this regard I believe is
Issue #145 (There is a subpixel gap between the main glyph and the border). This was already fixed in XySubFilter Beta2 (with some limitations regarding text overlaps), but not yet with xy-VSFilter.