Thumb-2 and Code Density
On a desktop CPU, instruction encoding is an implementation detail you can go a whole career without thinking about. On a microcontroller it is a budget. The STM32F411RE has 512 KB of flash and 128 KB of RAM, and that flash figure is one of the two or three numbers that set the price of the chip. Every byte an instruction occupies is a byte of product cost, multiplied by the production run.