Lifestyle tips

Mobility issues are different for everyone but 333 is still a pretty low number to be having it that bad. Maybe try getting some clothing that will help support your weight.

I agree that is a low weight, it really does sound like back issues. Have you talked with medical professionals about your back? (Yes, many will just be 'lose weight' but if you keep working at it you may get a useful diagnosis and advice)

Mobility depends on so many factors. As you said, it's possible for someone barely above 300 pounds to have severe problems. But there are other examples. The web model Boberry is at about 600 pounds and is still surprisingly agile. Can kick higher than her head, can run around with a thinner person on her back, and so on. It seems with lucky genetics, lack of other unrelated health problems, and good exercise, one can get to quite large sizes and still retain reasonable mobility.