For better or for worse, the new Honda Accord is classified as a full-size vehicle by both the rental agencies and the EPA. The new Accord is noticeably larger than the last model Accord, so it's not as small as you might be thinking, but I do agree that the Charger is a substantially larger car.
The problem with DTAG is that they are bound by a contract with Chrysler to feature Chrysler cars through the website. Chrysler no longer makes a true full-size car, and so the only options are the Chrysler Sebring/Dodge Avenger, which is considered a midsize, and the Dodge Charer, which in all honesty should be considered a premium. But because there is no in-between car (sized more like a Malibu, which other agencies consider fullsizes), the Charger defaults to the fullsize class. That leaves customers with the expectation that a fullsize is at least a Charger.
For the car class, DTAG's cars are usually larger than the competition. For example, here is Enterprise's car size chart:
Economy: Chevy Aveo
Compact: Chevy Cobalt
Intermediate: Dodge Caliber (considered a Compact by DTAG)
Standard: Dodge Avenger/Chrysler Sebring/Pontiac G6 (considered an Intermediate by DTAG)
Fullsize: Chevy Malibu
Premium: Dodge Charger, Pontiac Grand Prix, Nissan Altima (considered fullsize by DTAG)
Luxury: Nissan Maxima
So, rather than be mad you didn't get your fullsize Charger, you should be happy that you got a free upgrade to a premium Charger from a fullsize Accord!
(But do write DTAG corporate HQ and tell them their contract with Chrysler sucks and they need to fix it so their car model display is more in touch with what they actually carry and what customers should expect...it would make my life a LOT easier...)