DVRs need more space because of the hard drive. But the main considerations for cable box size apart from the actual number of components are RF shielding/proximity of components and ventilation/cooling. A smaller cable box would likely run too hot. It needs room to breathe.
here in Chile the setop box for VTR SD service is very little is the Motorola DCT 700 the HD service with PVR set top box is the Motorola DCX 3400 is not that big ,the good is that the set top box that they use are Motorola brand at least is better than the other satellites companies that use coship or huawei set top box for dish