The problem is, Hertz' car classifications are described by size, but the cars within the classifications vary greatly in size. Someone who is actually booking a car based on the described size may be disappointed if the car is not the size that the description implies.

This thread http://www.flyertalk.com/forum/showthread.php?t=743032 lists Hertz' various car classes. The C (midsize) class varies between Corollas and Ions (both of which are smaller than some of the B (compact) class cars like the Focus, Elantra, and Spectra) to Optimas, which are quite large. The Jetta is within the YC (special midsize) class, so it can be considered an "upgrade" over the C class, but it is more of an upgrade in terms of being more luxurious feeling rather than being larger, as it is smaller inside than most C or YC class cars.