Rockets subtract planet mass on launch (!), but spawn cargo based on its mass at the time of landing, with mass already subtracted.
For example, sending a rocket with 3000 kg capacity, on a 3 cycle mission to a planet with 3500 kg available resources and 100kg/cycle recharge, would return 2400 kg of cargo (800 kg mass available, but 3 cargo holds, each receiving 800 kg).
On the flip side, sending 20 rockets at the same time, to just one planet, will still produce mission time * planet recharge rate of resources, per cargo hold, as long as no missions are started while those 20 are ongoing.
Pause the game. Send 4 rockets with 3000 kg capacity each to one of the "orbit 1" asteroids, with 12000 kg of available mass. Wait for them to return. Notice that their cargo holds aren't full, but rather have about 3*166kg per cargo hold.