Actually you will see lot of macros defined to achieve small and modular functionality.
this makes your code faster to execute but the binary size will be bigger.
You will see this in kernel code more because we have very low stack area sometime may be one page(4096 bytes) of memory.
automatic variable declare and defined in function will occupy space in stack.
In application side C expert suggest to have functions because we have very big stack area and function adds readability
with easy debugging.
I hope this will help you in your kernel context.
Thanks,
Naveen