In adaptations of The Lord of the Rings, Samwise Gamgee is consistently portrayed as being overweight. In Peter Jackson's movies, for instance, Sean Astin was specifically asked to put on a good deal of weight for the role. In the earlier animated adaptations, the same is true.
Samwise (right) and Frodo, The Return of the King, 1980
Samwise (right) and Frodo, The Lord of the Rings, 1978
But I don't recall reading anything in the books that would support this image of Sam.
In fact, people seem to believe that all hobbits are obese to some degree, although the text seems to suggest only that they tend to become overweight as they get older. Samwise is among the youngest of the hobbits in The Lord of the Rings, so in the absence of other information, it would appear that he should be relatively slim by hobbit standards.
Do we have any reason to believe, based on Tolkien's own writing, either in the books themselves or elsewhere, that Samwise Gamgee was especially fat?