What's the difference in terms of system operation?
And why is it when I use small snowflake my a/c smells like feet?
the small snowflake will cycle the system on/off whereas the large snowflake will run it constantly. Your feet smell is probably related to the amount of humidity in the air, especially right before it is about to turn on again in the cycle (when humidity would get through the most).
there is bacteria in your evaporator.
"When moisture condenses on the evaporator, it’s supposed to drip drain away. However, very often the drain will become clogged and/or it won’t drain quickly enough. If water can stagnate inside the evaporator unit, this water grows mold and bacteria that causes your vehicle’s air conditioning system to smell — especially immediately after the system is first turned on."