knock down a full-grown adult and cause scrapes, stitches, even broken bones

Bolded the important parts.

Falling over for any reason can cause scrapes, stitches, even broken bones, regardless of the instigating factor.

An adult weighing three times as much of a goose doesn’t fall over because the goose is strong. They fall over because of being startled into losing their balance, which is a feat a sparrow or a mouse could also accomplish.