Each size helmet is supposed to be tested with an appropriate sized head form. There's no certification that says that any size helmet is safe for any size head. If a manufacturer says a helmet is certified for heads 52-58 cm, there's no knowing how it would perform on heads outside that range.
It's just as much a concern with helmets that are too small (which may lack required air space or cause the user's head to compress the shell over time), but it's more obvious when you're talking about wearing a helmet that's too big. Even if the inner retaining ring can be cinched down tight enough, there's going to be more play in the parts that connected it to the shell, so it hasn't been tested for its ability to stay in place during an impact or series of impacts.
ETA - this is why you can't get smaller street motorcycling helmets in the USA, even though they're readily available in Europe. The manufacturers have had the smaller sizes CE approved, but there's not enough market in the USA to get them DOT approved. That's why you see kids in the USA wearing MX helmets when they're passengers on street bikes - and racing in grey market helmets that couldn't be legally sold here for street use.




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The fabric is very lightweight so I'm not convinced of durability. We'll see.
