its more about concentration of force onto a very small area. the glass is tough but brittle, and it;s designed to shatter in a manner than doesnt leave dangerous shards. you use the hammer to shatter it, then pull it out of the way and escape
a larger hammer might work just as well, its just they want a smaller one thats easier to use inside the (wrecked) vehicle, and something they can install and secure right next to the window so a panicking (and potentially injured) passenger, working in a restricted space and poor angles, can pull it out and pummel the panel of glass to escape.
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