In Wallace's "Infinite Jest", he describes a girl as being "two hundred kilos if she was a kilo". Is she two hundred kilos or isn't she ... ?
The idiom "it's X if it's Y", where Y is much smaller than X, like
- "She was two hundred kilos if she was a kilo"
- "He's six feet tall if he's an inch"
- "It's twenty miles if it's a yard"
is an informal, emphatic way of saying I am certain that X.
You could think of it as a way of saying, for example, "I am as certain that she weighs 200 kilos as I am that she weighs at least one kilo (and since she definitely weighs at least one kilo, I am therefore certain that she weighs 200 kilos)."
See this discussion on English.SE: Origin of “he's 6 feet tall if he's an inch”