I don’t think it’s that she looks “fat” in comparison (granted, SMG and Emma Caulfield, especially, are too thin) but rather it’s the godawful unflattering outfits they stick her in. Tara looked good in the first part of the musical ep, when she was wearing the corset. I’ve seen pictures of Amber Benson as “herself”, and she looks average-to-slender in those.