The population of each planet is the market size^10. Since population gets exponentially larger with size, and both the produced and demanded resources increase with size, it would not be unreasonable to assume that resources scale the same way.
Because of this, individual production units are not worth the same. A size 6 colony has a population of "millions", and it has both a demand and base production of 6 food. That means that a production of 6 food is enough to support millions of people. A size 3 colony on the other hand would be home to only thousands, and would both produce and consume 3 food. Clearly, two size 3 colonies would not consume equivalent food to a size 6 one.
And if you got rich farmlands, that's a +1. A size 6 colony could now not only feed millions, but tens of millions. Got BOUNTIFUL farmlands instead? That's a +2, now you can feed hundreds of millions with your output of 8. Got an administrator with industrial planning on top of that? Now you can feed billions. Soil nanites on top of that? That's a +2, now you got an output of 11 and that's just ricidulous. Actually, at this point it all really breaks down and becomes very unimmersive again, but I suppose that is to be expected given that it's a simplified game system and not a simulation that is trying to be in any way realistic.
Without all these bonuses it makes enough sense, exponential growth means that higher outputs from a single planet are worth a lot more than those from smaller colonies, so you can't really count the total production unit numbers. And it makes enough sense that a planet that can feed millions with an output of 6 could feed a good number of other colonies that only have populations numbering in tens of thousands.
I think in vanilla the largest exporter of food is the LC capital with a population of tens of millions. That should be enough in food exports to feed a lot more worlds. If we're being generous, a size 7 world could have a population of 35m or so, but with enough food production to feed 50m. That could feed several other size 6 planets, and pretty much every smaller one besides, or another size 7 colony that is on the small side within that group.