I think the definition of big is smaller than that. Mine was "too big to fit on a maxed-out laptop", effectively >8TB. Our photo collection is bigger than that, it's not 'big data'.

Or one could define it as too big to fit on a single SSD/HDD, maybe >30TB. Still within the reach of a hobbyist, but too large to process in memory and needs special tools to work with. It doesn't have to be petabyte scale to need 'big data' tooling.