iDevice. 32bit or 16bit?

The iDevices contain an ARM processor. These processors support two instruction sets, the ARM and Thumb. Thumb mode uses a 16bit instruction and register set while ARM is all 32bit. Thumb mode typically reduces code size by about 30% compared to ARM. Most applications perform simple operations that don't take advantage of the 32bit instruction set and data type width. It is this fact that has lead silicon designers to create the ARM Cortex (an exclusively Thumb mode ARM) to reduce power consumption and manufacturing cost.

However, if an application makes extensive use of floating point code it may perform better in the ARM mode. Take for example, an application based on the Box2D or Chipmunk physics simulation engines.

BTW: Compiling for 32bit ARM mode in Abby Crabby only increased the image size by 3%.