Are you refering to the way in which both are described as being rather fat? This is certainly one of the key ways in which both are described. Let us remember that Red Sammy is described in the following way:
His khaki trousers reached just to his hip bones and his stomach hung over them like a sack of meal swaying under his shirt.
Hiram, in a similar way, is describes as being "a fat boy in black trousers and a red sweat shirt with a silver stallion embossed on the front of it." Both characters are therefore defined by their obese nature. I wonder whether there is this similarity because of the way in which Red Sammy is the character that introduces the whole concept of what a "good man" is with the Grandmother. This is of course one of the central questions that the story is based around as we consider the various characters and think of what a good man might actually look like. Red Sammy and Hiram, though clearly very different men, are perhaps linked or juxtaposed through their physical similarities to explore the ways in which humans can turn out to be so different from each other.