There is NO minimum width of an International Rugby League ground. The max width is 68 metres.
This is primarily because many English games were played on local soccer fields, hence they were lucky to hit about 60 metres wide.
Olympic Park in Melbourne is 62 metres wide for NRL games, and Carlaw Park in Auckland used to be about 58 metres wide for full Internationals.
So if there is no space left on a ground, having a narrower field is not an issue. The fact Fremantle appears to have done it deliberately and not utilised all the space seems a fair issue.