Probably the biggest factor that determines the size of fish that a river can hold though is the food supply. Big fish have big appetites and need an ever increasing quantity of food to maintain and even increase their bulk. A given volume of water can only produce so much food.
Of course, not all bodies of water are able to produce the same supply for a given volume of water. Rocky mountain streams provide very little in comparison to lowland swamps. The fast flowing water and lack of anchorage means that few plants can take hold and, because of this, there is little food up through the food chain. Mountain streams are also colder places and that affects growth rates too.
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Many species adopt a more predatory diet as they become larger simply because the value of this food supply is more in line with their needs. The Taimen of Mongolia is the world's largest Salmonid species. When young they live mostly on insects but as they get bigger (and they regularly get to be more than one metre long) they turn to a predatory diet consisting of fish, rodents, amphibians and even birds.
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An alternative to the predatory diet is the scavenger's diet. Many big fish around the world get to be the size they are because their particular habitat has a plentiful supply of some form of waste. The Goonch of India have become infamous after reports that they have been feeding on the remains of cremations from riverside Ghats. Goonch are a kind of catfish and are bottom feeders so it is likely that they would find any decomposing flesh in their territory whatever the source. We needn't take the choice personally. This is hardly the same as a rogue man-eating tiger.
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