Here's the extra info on the seat if it helps. It certainly looks wide and deep in the photo. It doesn't appear to be able to be used as a backless booster like similar models.
Meets or exceeds federal motor vehicle safety standard 213, no belts to rethread or parts to lose
16¼"L x 18¾"W x 29¼"H seat in up right position
12¼"L x 18¾"W x 23"H seat folded down
Deluxe Features
Height adjustable, with 6 positions for kids 38-57" tall
2 retractable cupholders keep beverages level
Shock-absorbing EPS headrest with side wings
Extra-deep, extra-wide seats
Belt-positioning booster requires no rethreading
Thick, triple-layer padding for comfort
Stain-resistant cover removes for machine washing
Retractable arm rests
Seat-saver base protects vehicle upholstery
Foldable, Adjustable Booster Car Seat Carry Bag
Nylon carrying bag, full length zipper and convenient carry strap
Perfect for traveling
Folded dimensions: 9"L x 8"W x 2"H
Open dimensions: 24½"L x 20½"W x 9"H
Handle: 25¾"L at smallest setting, 47½"L fully extended x 1½"W