That's completely valid but there's a world of difference between 300 sq feet and 500-600 sq feet per person. The McMansion is rediculious but that doesn't mean that the opposite extreme isn't also crazy.
Admittedly, my family of three lives in 1800 sq feet and we probably have at least 200 sq feet that only gets used a handful of days a year. I wouldn't want to go smaller than about 1300 sq feet for our family though.
I think it's probably similar for a lot of people; feeling that a smaller house is just right for them.
It's probable that the "tiny" part is a lot more reactionary to big suburban castles, and I'd posit that those into the tiny house fad are likely more interested in houses that are more compact/dense, than an actual tiny house.