I recently brought 5 very overweight (probably 15 kilos each - full of books)carryon bags from London to Melbourne without any of them getting a second glance. (Five people travelling with them, by the way.)

Perhaps I was let through because they were slightly smaller than the maximum size? Size being easier to guess than weight when you see people with them, I bought them hoping that this was the case. If they'd been any bigger, it would have been an exercise getting them into the overhead locker.