Notably even an economy compact car like the Corolla can have keyless entry, I think it was an option for the Corolla starting in ~2007 unless I’m mixing up models. So it was not just mid-tier cars back then, but also the smaller & more affordable ones.
Whatever file format I use them in is also how I back them up, I backup my entire desktop’s and laptop’s data to an external hard drive and an online service provider. I’m sure a compressed format would be more space efficient but that would take much more time given my use case.
In the case of my laptop it runs Linux and the filesystem I use supports “transparent compression” (almost all contents of the drive are compressed with zstd), so I’m guessing any of the ROMs on there will have already been compressed as nuch as they can (but I’m not knowledgeable enough on the file format specs)