You'd think, right? Luckily the ARM dudes came up with Thumb-2, so most (all but 6) of the Cortex-M0+ instructions are actually 16 bit, not 32. (Nice chart at http://en.wikipedia.org/wiki/ARM_Cortex-M .)

But what's even more interesting is to actually compile apps and compare code size. Miro Samek did this for a couple of RTOS-like framesworks and compared across a mix of 8, 16 and 32 bit micro architectures (PIC, 8051, M8C, 68HC08, AVR, MSP430, M16C, ARM7 Thumb, and ARM Cortex-M3 Thumb2). For his test, the MSP430 was the most dense, but Thumb2 wasn't far behind. (http://embeddedgurus.com/state-space/2009/03/insects-of-the-...)

Of course, there are 4 bit micros out there...