In nature, it usually pays to pack on the pounds, especially if you're male and competing for a female. But a treasure of a paper explains why male anacondas have to fight for their ladies while staying small.
If you have to fight to get a chance to mate, everyone who reproduces is going to be relatively big. Skill is great, but size and weight are key. This is why species in which males fight each other to claim a mate produce males that are bigger than females.
This is part of sexual dimorphism; the term for the fact that males and females of the same species can vary in ways other than their genitalia. The most obvious case of sexual dimorphism is that of the peacock. Flamboyant males woo their drab females, and the prettiest male wins. Males and females within a species can vary all kinds of ways.
Humans are relatively homogeneous. Males are on average only about 15% bigger than females (a small percentage for a primate) and there's a slight difference in body shape. Compare this to, for example, the anglerfish. Anglerfish males are so much smaller than females that for some time biologists mistook males for small parasites on females' bodies. But then anglerfish males don't have to compete for females, so they don't need to be big.
This is also true of most male snakes. Male snakes tend to be significantly smaller than female snakes. This isn't unusual in and of itself. There is selective pressure on females to be big; larger females produce more and bigger young than smaller ones. Large females are also more likely to make other animals their prey—- especially if they hunt by strength. When female snakes need to mate, they put out pheromones and the male snakes come to them.
Male snakes are in a different position. Most male snakes don't go into combat with each other to win females. Their challenge is traveling the long distances necessary to find a female, which not only saps their energy, but makes them vulnerable to predators. If they're small, they're harder to spot, and don't have to move a large bulk over long distances. Smaller snakes also reach sexual maturity faster, and get a chance to mate early in life. There are a lot of reasons for males to stay small.
A few snakes, however, diverge from the norm. Certain grass snakes, and the green anaconda, have mating rituals that include males competing with each other to mate with a female. These snakes form "breeding balls" or "breeding aggregations." Essentially, a group of males wind themselves around a female and fight each other to get to her cloaca. Sexy, I know.
And confusing. These male snakes are still much smaller than female snakes. Why would they stay small if only the biggest and strongest of them gets to breed? What's capping their size? A group of researchers think they know after they observed a captive anaconda breeding aggregation. [The paper describing it is worth reading, if only for the caption under the picture of the breeding ball. It reads, "Mating aggregation of anacondas involving a very large female (Ashley) and 11 males." They named the snake. Lord love zoologists.] In the breeding aggregation, a large female was being courted by males. A large male came near. A group of males broke off from courting the female snake, and began trying to mate with the male snake instead. This, understandably, interfered with the male snake's ability to land the female.
This may be why male anacondas (and some grass snakes) are so bafflingly small. When the snakes form a breeding ball, all the males line up with their heads near the head of the female, and their tails alongside her tail. They search blindly for her cloaca with their tails. In the confusion, only size and girth lets them know if what they are slithering over is a male or a female. Once males get too big, other males start mistakenly trying to mate with them, which vastly decreases their chance of getting a female. Confused snake orgies keep males small.
The world is a beautiful, weird place.