In my opinion, there isn’t any. What’s a compact today might have been mid-sized 20 years ago.
Sure we can break out the measuring tape, and get an aggragate of every car’s dimensions. But for me, I let the measurements matter when they need to and base my understanding of car sizes into the following:
City
Sub compact
Compact
Mid-size
Large
Full-size
It’s flexible enough to allow for things smaller than City cars and things larger than Full-size cars, but for now, I stick to the above.
I also pretty much let the automakers dictate what they say their car is and for the last 15 years it’s worked for me. I don’t overlap the segments, IE: the Cruze is not a compact mid-size, it’s just a fat compact – a compact as designed and engineered by GM.
That “fattness” of the Cruze is just an attribute of what GM felt was best for their compact car offering in North America. Arguably, it might not have been the wisest decision for them, but that is what GM calls a compact and it fit with my system.
For me, my system works as I can easily relate one car in a segment with another in the same segment. I don’t include price because (for example) in compact segment, you can spend as much as you want and still have a compact car, be it a Kia Rio or a M3. There are things a compact Rio and M3 can both do, and something that only one can do that the other can’t. The difference does come down to need and desire. Some would say that the Rio is the better car because it does X, Y, and Z better than the M3 and that for their money, that’s what matters.
I’m not saying my system is bullet proof, but that’s how I make sense of car sizes in their respective segments.
