It may be counterintuitive, but convertibles are usually heavier than coupes, due to the added bracing and stiffening that compensates for the lack of a roof.

Miatas are pretty small, but I agree GP is right to lament the lack of lightweight vehicles in the US. I've read the average European car weighs ~1000 lbs less than its American counterpart. I personally feel gross driving around in heavy vehicles, like are people really so insecure they feel the need to haul all this extra mass around, and ignore the repercussions at scale?