Increasing T1 mex to T2 triples the mass (200% increase). Putting storages around a T2 mex boosts the mass by 50% (50% increase). Increasing T2 to T3 triples the mass again (200% increase).
So if you just look at the succession of stages, it might look like an exponential increase in income.
But that ignores that the cost also scales. In fact, the cost scales faster than the benefit. If you just compare the cost to benefit, it scales linearly (slightly decreasing).
If you take into account that as you upgrade mexes, you increase your mass income, so you have more mass to invest, then you will see that it scales in a quadratic manner.
If you can afford to invest a constant amount of mass into upgrading mexes, let's say 8 mass/second, and you invest any new mass you get from upgraded mexes into making more weapons to fight the enemy, then you will scale linearly.
If you invest all of your income into eco, and skip making weapons, then you will scale quadratically.