That's like saying lead balls are lighter than rubber ones because they're smaller.

The value at risk is the size of the investment.

Not unlimited. Your losses shorting a stock are capped at the price you borrowed it for (though technically it could be unlimited if you have to continue paying borrowing costs if you can't buy a share and close out your short [1]).

[1] https://www.bloomberg.com/opinion/articles/2018-04-11/-go-to...