They showed him as a kid, he wasn't all that big. Just a typically looking kid. The ultimate problem with Bertil was he was massively insecure. Like he was trying to defeat the little boy inside of himself, when really he never grew up emotionally.
the part where his friend told the story of coming to his house depressed about how he looked and posing in his friends mirror
"damn bertil you look great"
"i only look great in your mirror not in mine"
that's some serious mental illness
