The pop growth bonus is awesome, if you're the one getting it. Probably a little too awesome. It's the massive initial colonization phase that wins it. If a planet has 3% pop growth, it takes 23 turns to double its population. With 70% bonus, it takes 14 turns. In 23 turns, it actually has 3x its original population. BUT: you're at 100% morale during the initial colonization phase. That means 2x population every 11 turns vs. every 7 turns, and every 11 turns the guy with 70% is gaining 50% more population than the one without.
Your taxes, on the other hand, are the square root of your population. The extra population is no big deal if you're square rooting it, right? Let's do a little calculus. Take the derivative of sqrt(population) to get the rate of increase in your taxes per population: that would be 1/2*pop^(-1/2). For values > 1, sure, your tax growth rate is slowing. That's not the problem. For SMALL values, < 1, the rate of tax increase is actually INCREASING, at a GREATER than linear growth rate. And that is what's happening during the initial colonization phase--you have an exponential population growth rate, multiplied by a GREATER than linear tax growth.