Its the same reason intake tubes and valves on engines are almost always larger than output/exhaust pipes/valves- you can "suck" as hard as you want, but ultimately you are limited to the force of atmospheric pressure to feed the air into the end of the hose. The output/exhaust is forced out at high pressure and can pass a lot more gas through a smaller port, in less time.

Lower intake pressure is the reason the suction/output hoses on an AC compressor are different sizes too (suction is larger), but that system is sealed, so atmospheric is not the reason there.