There are different buses for different budgets. If you go for the cheap, you get little leg room, seat width, or privacy. Pay more and get more.
Willer New Premium Seat
http://travel.willer.co.jp/seat/new_premium/
Single aisle seat one way 7100 yen. seat pitch 117cm, width 50cm
Willer Executive Seat
http://travel.willer.co.jp/seat/executive/
One way 11000 yen, seat pitch 107c m, width 80 cm, 3 seat mini-cabin
The ordinary class bullet train seats have good leg room but not width if you are large horizontally. One way 13710 yen.