in this scene, Mufasa is seen sitting with Rafiki during a hug.
in this one, Simba is seen sitting with Rafiki by the water.
Since Rafiki standing at full height is still not as tall as Mufasa in the first scene, and he's got to bend over to get to eye-level with Simba in the second scene, I think it can be concluded that Simba is at least a little bit smaller than Mufasa even though Simba is sort of slouching in the second scene. The only question remaining is, why such a difference?