Astrax, if you follow independant game developers, you'll find that it is the case. Go look at Cornutopia, for instance. Same thing there, and its a whole (successful!) company of 1.
What you find is that successful independant companies stay successful by being realistic in their sales projections. For instance, Cornutopia projected they'd make $10K off its Flatspace game, and cut their development budget to be $5K in accordance. At a risk: losing that $5K and the time to make the game. Expected reward: $10K. (Flatspace did a bit better then expected, letting Cornutopia more easily fund Flatspace 2

). If you go check up on the history of GC1, you'll find a similar story. Star Dock expected to sell about 10K worth of units of GC1. Thanks to MoO3 being the total flop it was, GC1 really sold out. But Star Dock couldn't bet on MoO3 being a failure. It couldn't bet on when MoO3 was coming out. And if MoO3 had been a smashing hit, Star Dock would have only had typical sales of the 10K units of the game. That's why Star Dock budgeted GC1's develop as they did... so they'd weren't risking the companies future on how big a hit GC1 was.
Star Dock has budgetted more for GC2, AIR. But then, they have a lot of pre-orders (so they know what their guaranteed sales are). And then there's the fan base and other elements. Star Dock expects to sale a fair bit more then 10K of units of GC2, but they still cannot afford to sink in the $10 Million that EA backed studios would spend making a MoO4, for instance. Brad wants to keep feeding his family just as much as everyone else, so he just can't go mortgage his company to sink in such large budgets into making GC2.
As I said, go check out the indie game market. You'll see its the way they all the successful companies work. They budget for small to moderate sales, and use that as their baseline for how much to invest and risk. Sometimes, they sell as expected. Sometimes, they are breakout hits. A lot of time, they fall short. But as long as they don't fall too short, the indie can cover their costs and have a bit of pocket change left over.
You want GC3 to have a bigger budget? Get all your friends to buy GC2 and pre-order GC3 when Stardock starts collecting pre-orders for it in a few years. Then GC3 will be a meatier product!