The amount of sound reflected by a fish is measured by its target strength (TS).
For the same echo intensity (amount of energy in an echo), there can be many small fish or fewer large fish. Some fish reflect sound better than others, so even for the same size of fish, for any one echotrace, there may be either be many of one species or fewer of another.
Different Fish have Different Target Strengths
Some species, such as cod, have a high target strength. Others, such as mackerel have a very low TS. Fish such as herring have an intermediate TS.
The TS is determined experimentally (in cages, for example) and depends very much on the physiology of the fish. The most important factor is the fish's swimbladder. A fish swimbladder is full of air and air reflects sound much better than anything else underwater. So a fish with a big swimbladder, like cod, will have a high TS.
There is still uncertainty about the exact value of fish TS and how it is likely to change with depth, season and the behaviour of fish in the wild. Experiments to determine TS are notoriously difficult: as long as acoustic surveys can deliver a valid index it is difficult to justify the large resources required to fund research on TS.