Most of what I know about the Leicester fat man Daniel Lambert comes from Dr. Jan Bondeson's
The Two-Headed Boy and Other Medical Marvels.
This article synopsizes his life.

He was born in Leicester in 1770, and while large, didn't grow exceptionally fat until he reached adulthood. He was known overall as having a generally kind and generous nature. As a youth, he taught other boys to swim in the local river, being especially buoyant. He apprenticed to an engraver and then took over his father's position as keeper of the Leicester "bridewell," or private prison. While a large young man, it wasn't until he adopted the sedentary life of a warder that he began to grow exceptionally large.
The two events might have been coincidental, though, because Lambert continued to ride, hunt, and swim until five or so years before his death. Contrary to the common wisdom, he neither drank ale nor was thought to "over-eat."
As prison-keeper, he was renowned for his kindness. Prisoners were given cots instead of being made to sleep in straw and filth; the prison was whitewashed regularly, and the prisoners respected him. When the local prison was merged with the county one in 1804, Lambert was pensioned off, and a year later he began to exhibit himself in London. He weighed about 330 kg, or about 725 lbs.
Lambert didn't like putting himself up for exhibition. He charged a fair amount to keep away the idle curiosity seekers, and received people in his own London home. He managed his own money and didn't use a handler or manager.
While it's true that in the late 18th/early 19th century, a "stout" man was considered handsome and healthy, Lambert's unusual size did engender some impertinence and rude remarks. A Swedish officer referred to him as "this hideous mass of flesh." One woman asked him rudely how much his capacious coat had cost, and he answered, "If you think if proper to make me a present of a new coat, you will then know exactly what it costs."
Lambert never married, but according to Bondeson, when he was on exhibit he had many women visitors, who spoke approvingly of him.
Without any previous illness, he died suddenly at the relatively young age (even for that day) of 39. Apparently he got up to shave, complained to a servant of shortness of breath, and collapsed.
Bondeson spends too much time wondering what made Daniel Lambert such a man of size, and assuring us that he could have been "helped" by gastic bypass surgery. No doubt Lambert's life as an extremely fat man was far different in an era where his size was not yet "medicalized." He was treated as a wonder, a marvel of nature, and sometimes an object of ridicule or smart remarks, but not in general an object of pity or revulsion.