Selectively inflatable airbag side chambers

An assembly includes a vehicle seat. The assembly includes an airbag supported by the vehicle seat and inflatable to an inflated position. The airbag includes a main chamber, a first side chamber, and second side chamber spaced downwardly from the first side chamber. The assembly includes a computer including a processor and a memory storing instructions executable by the processor to determine a size of an occupant seated in the vehicle seat and, based on the size of the occupant, selectively inflate one of the first side chamber from an uninflated position to an inflated position or the second side chamber from an uninflated position to an inflated position.