The further back the metatarsus goes in this context the less the angle between the femur and tibia not the more the angle. In the diagrams shown here the correctly angulated dog has an angle between the femur and tibia of 135 degrees [105 + 30] and the over angulated dog has an angle between the femur and tibia of 115 degrees [105 + 10] When the tibia is too long the angle that changes is the angle between the tibia and the horizontal plane, the femur starts to become a little vertical to the horizontal plane but not that much.
The general observation that people see and tend to comment on is that an overlong tibia, an overlong lower thigh, creates an unstable hindquarter and this is true of course, but over angulation also lowers the hip and knee position, that is the hip and knee comes closer to the ground in stance and in movement. This lowered position of the hip and knee has been further increased by the breeds development to a downward bend to the lumbar spine - now a 'norm characteristic' of the GSD show dog and for many people not even seen!
Other than its aesthetics what does this mean in the terms of locomotion?
The higher the hip position the longer the stride, the lower the hip position the shorter the stride and the more the angular articulation that is required by the tibia and femur to execute each stride. Assuming equal pelvis angles and lengths and equal femur and tibia lengths a dog with a low hip position needs to make more strides than a dog with a high hip position to cover the same amount of ground.
Putting aside the impeded forward foot step down position, the impeded rear foot lift off positions and the rear pastern getting parallel with the ground at its most forward step position, the increased angular articulation and increased distance that is required of the femur and tibia in the lower hip/knee position manifested in over angulation is to the detriment of endurance. This is because overangulation consumes excessive energy and impedes manoeuvrability. As the tibia gets proportionately longer, in the vast majority of cases in increments relative to tibia length energy consumption and hock instability increases. Contradictions to the fundamental needs of a working trotting endurance dog.