In Proportions and some other mods, at least ground combat probably won't wipe out all the attackers of a strong "beachhead force" before the later transports can drop troops.
In unmodded, one round of ground combat usually wipes out one side or the other, but the attacker can usually fit more troops on one transport than the defender will have room for on their planet
, if they even have any serious ground defense, which tends to be rare in unmodded.
PvK