Luke:
Another issue: planets with a PQ less than Earth's act as speed-brakes on your economic development. Because their populations tend towards lower morale, quicker, than Earth, they will minimize the tax rate you can set Empire-wide - and thus restrict the % amount you can devote to spending.
You can see this by starting a trial game (any settings/picks will do, although avoiding pop/morale/income bonuses will allow the results to be seen more clearly; save at turn 1) and not colonizing anything. Simply focus on running Earth as efficently as possible, maximizing taxes & spending while maintaining morale at 80+. Take note of your progress every 6-12 turns.
Then reload the game and start again, colonizing a sub-Earth PQ planet of your choice. Try keeping pace with the income/morale/spending/build rates established in your first try. You will immediately see that you can only support the colony by decreasing Earth's settings below optimum values, i.e., by "devaluing" Earth, your strongest starting asset.
You are far better off limiting your initial colonization efforts to planets equivalent or superior to Earth. Once Earth is built up, then you can consider a second colonization wave of those planets PQ15+ which you passed over the first time.
~SDC~