That's not precisely correct. I travel frequently on the Shinkansen with an Away "The Medium" (https://www.awaytravel.com/suitcases/medium) stowed on the overhead rack above my seat. You don't need to reserve seats with extra luggage space for anything under 160cm in total linear dimensions. To give you an idea, the Away Medium is 139 cm total linear. And that bag is WAY bigger than anything allowed as carry-on by airlines.