'That's peanuts', we say when we're talking about piffling amounts, drops in the ocean. Or we say 'that's peanuts' if we are confirming the presence of peanuts. Or we're making a distinction between them and chickpeas, which look a bit like peanuts (though, really, we shouldn't because chickpeas resemble more the hazelnut).
What amount is considered peanuts has been very subjective up until now, now that we have organic peanuts that only wealthy people without an allergy can afford.... (to read more, please go to www.tinybangtheories.com)