While the Air Carrier Access Act may apply to air travel as well, the ADA also applies to public transportation. Here is an excerpt from their official website:

The Title II regulation defines a physical impairment as any physiological disorder or condition, cosmetic disfigurement, or anatomical loss affecting one or more of the following body systems: Neurological, musculoskeletal, special sense organs, respiratory (including speech organs), cardiovascular, reproductive, digestive, genitourinary, hemic and lymphatic, skin, and endocrine.

...However, at some point, obesity itself may be considered an impairment. While the point at which this occurs must be considered on a case-by-case basis, it is generally accepted that morbid obesity, which is defined as body weight 100% over normal weight, is an impairment. In addition, an obese
individual may have a related physiological disorder, such as hypertension or a thyroid disorder, that either causes or is caused by obesity. These physiological disorders are, by definition, impairments. ...

As you can see, morbid obesity is an impairment. As the FA on duty, I am to ensure that all persons in an exit row meet certain criteria. While it may be hard to believe that an obese person can perform these functions, I cannot discriminate if that person tells me he can open the exit. Furthermore, an obese person with no other existing conditions (ie. use of cane or wheelchair) does meet exit row seating criteria. Although I may empathize with you, these standards are implemented by the FAA, CAL, and other organizations.