It's not photoshopped, it's the outfit, the "glitter line" on her hip is very "far" so from this point of view + with the light you have this feeling of a huge ass. By the way i love Britney's face, so cute :D
See this non-Gaga related post is apparently acceptable because it's poking fun at Nicki. Yet if I was to post a NICE picture of Nic I would be beaten down with abusive comments because "This is a site for Gaga!". And don't say that wouldn't happen because it has.