Convertible car seat size and safety

Are all convertible seats the same, safety wise? I assume they all have to pass some sort of test—? I bought the Graco Slimfit because I have a smaller crossover SUV. But now I’m looking at other larger seats. They’re bigger, does that mean they’re safer? It’s hard to tell because they pretty much all make the same claims and have the same weight/height limits. I don’t want to spend the extra $150-250, but if they’re safer, I will. (Baby is still in his Chicco Keyfit infant seat.)

Any info on safety that I’m missing? Or does anyone have the Slimfit and have a review?