You need to remember to grab new colonies too. There's going to be a point of diminishing returns when sending arcs to existing colonies, so you need to strike that balance of bolster versus new.
This is true. For the nonce I'm just colonizing every planet within range.* The planets that are closest to my home world get the benefit of new arcs: ideally the range is such that I can build and send an arc or two to the new colony and have it arrive within a turn or two. Beyond that time-range I'd assume it's more effective to maximize colony worlds rather than colony populations.
*I want to have a nice tightly packed cluster of worlds under my control rather than a far-flung empire.
