Characteristics
A "tomato can" is usually a fighter with a poor record, whose skills are substandard or who lacks toughness or has a "glass chin." Sometimes a formerly successful boxer who is past his prime and who has seen his skills diminish is considered a "tomato can" if he can no longer compete at a high level. When referring to a distinguished fighter, opponents with passable careers who simply aren't at the same level can also be considered "tomato cans". Such an individual is an attractive opponent if his name still carries prestige but his diminished skills make him an easy conquest.
Most fighters who are considered "tomato cans" are heavyweights, because at lower weight classes one must maintain a certain level of fitness in order to make weight, whereas a heavyweight who once fought at a trim 205 pounds could conceivably gain 150 pounds and still fight in the same division.
One characteristic which may account for the use of the "tomato can" metaphor for a bad boxer is the tendency to leak "tomato juice" (i.e. blood) when battered.
"Tomato cans" are similar to jobbers in professional wrestling in that they serve to enhance the stature of someone the promotion uses to draw a crowd.
Read more about this topic: Tomato Can (sports Idiom)