they were still technically human as in breedable. it would be like a great dane, normal humans are like chihuahua up to alsatian. the difference in size is almost double from there up. that's why they called them giants.
it is hybrid vigor that made them so dangerous. they could throw around normal humans like we can throw around 2 year old children. and imagine how much food they needed for such a big body, i mean like, one cow would last a nephilim like a week, where it would feed a whole family for a season of normal modern humans.
out of all the crazy sounding stories, the ones about giants are the least crazy. i mean, it's not like gulliver's travels but a man at 12 feet tall is scary as fuck. even if he's a skinny weakling he's still gonna be impossible to defeat without luck and skill. they were already becoming very rare by the time of David, most likely goliath was only like 10 foot tall. if you are a psycho bitch and good at strategy and you got guys like that, it would be trivial to dominate ordinary homo sapiens. even neanderthals were a problem for sapiens. but these guys were something else.
i might be misremembering, but i think they actually found bones of an extraordinarily tall human a little while back. the angel people were not so ridiculous, more like 7-8 foot tall.