Why does a kangaroo look very muscular?
Because they very much are.

Kangaroos are some of the only marsupial mammals that active work out, more specifically the males in order to compete for mates as they are very social and live in groups. Like in many groups of organisms such as deer, males complete with each other to become the dominant alpha, giving them exclusive access to mating with the females of the group.

In order to be strong enough to fight others, males kangaroos will as youngsters spar with their mother and other younger individuals. They even work out by putting their hands on the floor and lifting their body upwards to build muscle strength in the arms. Combine that with their heavy intake of protein as a result of eating a lot of grass per day and you get Roos that look like this: