Buick Regal Sportback (2018) is 45 mm (1.77 inch) longer and 2 mm (0.08 inch) wider than Kia Optima (2015).
Kia Optima (2015) is 10 mm (0.39 inch) higher, 197 kg (434 lbs) lighter and has 40 liters (1.41 ft3) bigger cargo space than Buick Regal Sportback (2018).