Your Apple Watch is getting a whopping 1.5 watts... your MacBook Pro is getting closer to 65 watts. It will charge slower than your MacBook Pro. You're MacBook Pro( even when turned on ), has an effective charging rate of almost 28 watts. That's massive, considering it's just a USB C cable. At least, IMO.
Actually, not all true. A battery size is based on the capacity of the battery. The capacity of the battery is determined By the discharge rate. Effectively the iPad Air 2 has an 8 thousand mah battery while the MacBook Pro 13 has a 4300 mah battery due to its higher voltage. Granted capacity wise you are in fact correct.