It always goes up.

This is THE limiting factor for the mono. They are otherwise very strong. You pop is very limited. You can get around this by annexing planets, or by intercepting colony ships (wait till another empire sends colony ships somewhere then teleport a pop to that planet before they get there and you can "steal" the colony pop) but the latter practically requires pausing the game so is non-viable for multiplayer and the former is more late game. Otherwise focus on getting good money and production capacity and you should be able to outpace the growth.
For production, I usually use a local asteroid field (+50% labor) planet with a lot of space and/or moons and import titanium, supercarbons, iron (+30% support ship production), aluminum, and ideally hydrocarbons (+50% labor).
For money, focus on colonizing T1 textiles/electronics and other money producing planets to export to your T2s for level up. There are no % bonuses to money so you can safely export them to any planet you want to level. I also attach 1 morphic materials from asteroids to each of these T1 planets for an extra 150k and to fill up their pressure cap.

FTL energy can be a limiting factor in the early game but goes away if you have an FTl planet and/or enough gates and reactors. The anomaly scanning reward of a free gate/+1 FTL energy generation is also very good for you (note: these free gates do not use your FTL production or have a maintenance cost that I can tell)