As it's been mentioned in other threads, the reason for this change is that Enterprise (which now owns Vanguard/Alamo/National) classifies cars based on cost, not size.

An expensive small car will be classified in a higher class than a cheap large car.

Focuses and Cobalts are more expensive than Kias, so they get classified in higher classes.

Not very convenient for the customer, and I'm certainly not defending this practice, but that's the logic behind the move.

When renting from Enterprise, they are so often sold out of smaller cars that you'll end up getting a larger car as a free upgrade, regardless of what you book. With Alamo and National's much larger fleets, I doubt that's going to happen as often, so keep this in mind when reserving a car--you might want to book one size larger than you absolutely need (i.e. if you have four adults, book a fullsize so that you at least get something that a normal person would consider a midsize).