I see where you're coming from, nobody thinks that the performance was flawless but literally every celebrity has unflattering pictures and there's a reason why we call them "unflattering" - meaning, they don't capture the person in the best light and aren't necessarily an accurate representation of what the person really looks like.
I mean, we've all seen Beyoncé slay at the Super Bowl and she didn't look like this:
Taylor Swift doesn't actually look like this in real life either:
My point being that Britney's appearance at the VMAs that year was blown way out of proportion. Everybody has their good and bad angles, and if you capture someone dancing and performing from a weird angle you can easily make them look bad. Of course, the fact that she wore extremely unflattering clothes and her hair looked bad didn't help the cause, but she was nowhere near fat.