The DeepCool laptop cooler I own has larger dimensions than a newer model, but the newer model can support a larger laptop. Why is that the case?

The actual dimensions of a laptop cooler are not indicative of how large of a laptop it can support. Take for instance the X6 (380X295X24mm); it is larger than the X8 (381X268X29mm) in one aspect, but the X6 was designed to support a 15.6" laptop (a 17" laptop can be supported, but it is not the size that was intended for this unit). The X8 was designed to support up to 17" notebooks.