The phrase "small backseat" refers to a limited amount of space in the back area of a car where passengers sit. It means that the backseat is not very roomy or spacious. Full definition
They will also say, «You wouldn't believe how insanely small the backseat is in this $ 215,000 car.» (automobilemag.com)
It is a brilliant idea in an era when most reasonably - priced sports coupes tend to offer smaller backseats and less and less cargo capacity. (roadandtrack.com)
The interior's only flaw would be a very small backseat and the lack of cargo space. (automotive.com)