Yeah, I'm curious if a Camry is mid-sized what a full size is. Taurus? Those things are absolutely massive. My Ford Focus feels big, honestly, but maybe that's because I live in a relatively urban area with pretty tight street parking?
Car categories have agreed-upon definitions; what rental companies call them is usually a size up or two from the manufactuer's (canonical) names.
For Toyota, the Yaris is sub-compact, the Corolla is compact, the Camry is mid-size, and the Avalon is fullsize. For Chevy, the Spark, Sonic, Malibu, and Impala fill the same roles. For Honda, it's the Fit/Civic/Accord (they don't make a full-size). For Nissan, it's the Versa/Sentra/Altima/Maxima.
One time I had the option in the lot between a Malibu and Impala, and figuring out which one was "better" in person was pretty tough. The overall length difference per Chevy is 7 inches, which seemed largely irrelevant in person. I guess if it all went to the back seat leg room then you have a distinction for a certain market.
For reference, Sonic can be up to 35 inches shorter than a Malibu (depending on style).