How does this compare size wise to the old Big Kid booster?

Hi Stacey, thank you for your inquiry, the old Big Amp Booster is a very little bit wider than the Graco Highback Booster, I would say they would be a very similar fit in your vehicle. We hope this helps. if you follow the link https://www.babyonthemove.co.nz/store-locations/ this will take you to our online store finder. Here you can find contact details for your nearest Baby On The Move Store where one of our team experts will be happy to assist you in your product search and help answer any further questions you may have.