Thumb is about encoding instructions within 16 bits; it's not about the addressable memory, and it's not about running parts of the CPU in idle - that's an orth

Thumb is about encoding instructions within 16 bits; it's not about the addressable memory, and it's not about running parts of the CPU in idle - that's an orthogonal issue. The main advantage of a 16 bit instruction is that it is twice as dense in the instruction cache, reducing cache misses - which are extremely expensive in terms of power consumption.