"heavy" instead of "big" has not been accepted
It should be 'heavy' indeed, big would be 'kubwa'
If heavy means nzito then why is it accepted in this particular context?
Maybe it's because that's what the English phrase is? A lot of idioms don't translate directly but have an equivalent in other languages, so the Swahili could very well be "He has heavy bones" and it's just not directly translated in English.