For example, the Cessna 172 started production with the Continental O-300 145hp engine but today sells with the IO-360. Granted that engine is more "modern" but 145hp with the O-300 still feels too little for such an airplane.
The Robinson R22 started with the O-320 and eventually jumped up to O-360.
Do the designers not feel the lack of power when test flying the aircraft? Are we getting fatter?
The Cessna 172 first flew in 1955.
The Continental IO-360 was FAA certified in 1962.
The math is pretty simple.
Yes, we (Americans, at least) are getting fatter, but that still doesn't allow for use of an engine that isn't yet certified.
It's the same for GA as well as commercial aviation. Improvements in fuel efficiency and power tend to lead the manufacturers to want to certify existing airframes for new power plants because that's less expensive (testing, certification, pilot training, etc.) than building a whole new aircraft around a new engine.
Older planes were simpler and lighter, in part to make certification easier and in part to minimize the selling price and therefore maximize volume.
Once that minimalist plane hits the market, though, customers invariably ask for things to be added, which nearly always means more weight. (Indeed, more weight itself is often the request, either payload or fuel, for airliners.) This means you will need a larger engine to maintain the same performance. And, of course, some customers will ask for more performance too, which means a bigger engine and more weight. It’s a vicious cycle that spins upward—and so does the price.
Such variations and advances are much easier to develop and certify once you have the certification and revenue stream from the minimalist model, though, so you can’t really skip that stage.
Increasing engine size allows improvements in performance. This can be used to open up new use models for the same or very similar airframe (putting 150HP in a piper cub, for example). It also supports practical use of seating capacity with full tanks in an existing airplane or safer operation at high & hot conditions or quicker climb outs, etc. All of these are valid selling points that make a given model of plane more attractive to a potential buyer. And if the buyer prefers the smaller engine for some reason, he or she can always buy the previous year's model on the used market.
I think the idea "Cessna put bigger engines in the C-172 as bigger engines were certified" is a bit simplistic.
Compare an early model Skyhawk to a late model - they have very different empty weights, which is reflective of how the basic design got heavier over time. Electric flaps, g-rated seats, seatbelts (and now airbags), more avionics, different wing shape, a vacuum system, big square dash, new landing gear - the newer ones are very different aircraft. You can tell if you fly in them too, the older ones are not underpowered at all, if they're not overloaded. A new 172 with G1000 weighs over 300lbs more empty than a 1956 model did, yet the 1956 model has a bigger useful load despite having 35 fewer HP.
Here is some data I pulled from the 1956 model handbook vs the S (G1000) model POH:
Model Max Weight Empty Wt Useful Load HP Wing Ld Climb Takeoff Gnd Roll
1956 2,200.00 1290 910.00 145 13.8 660 1650 725
P/G1000 2,550.00 1663 887.00 180 14.7 729 1186 693
Takeoff distance is to 50', performance all assumes max gross wt. sea level, no wind, 15C.
I think it is especially important to look at the useful loads here. If Cessna were upgrading the engine because "we're getting fatter" they would have increased the useful load. Instead, the engines evolved over time - as did the airframe and equipment - as one cohesive design, while useful load stayed more or less the same (actually, it went down slightly). It is a back and forth related to technology (1956 vs 2010!), and the additional engine power really is just enough to keep the design competitive (power to weight ratio on the 1957 model is 0.066 vs 0.0707 on the current model, which is part of why the distance over a 50' obstacle is so different, but the other numbers (ground roll, sea level climb) are fairly similar.). In fact, I would suggest that an early 172 and a late-model both feel "underpowered" compared to a mid-70s N-model, when the design was still light but the engine made 160 HP. With a power to weight ratio of 0.070 and a wing loading of 13.2, it climbs at 770 fpm at sea level, better than either of the other two.
In general, the smaller the engine the less challenging the initial development and the cheaper and sooner the new airframe will hit the marketplace.
Early adopters provide valuable feedback, while the sales force soon get to know what extras the market is most desperate for. Meanwhile engines are also evolving and market expectations advancing. The designer then better knows the weaknesses in the Mk.1 to fix, the strengths to exploit, and the engine powers to aim for.
Also, the low end of the market is most sensitive to price, while the higher end is most sensitive to performance. So it makes sense to first launch down to a price with low performance, and then introduce the subsequent model with the latest and greatest engines.
For example when de Havilland specified the original ADC Cirrus for his forthcoming Moth biplane, the 70 hp Cirrus Moth was perfect for market expectations. Two years later he had made enough profit to develop his own Gipsy engine and the market now demanded nearer 100 hp in the Gipsy Moth.
