This is definitely true for cars, which is why I don't get the demands for a car list as large as GT6. They only reached that number by reusing very old models. Let's not continue to use those old models just to make the number bigger.
However, it isn't true for things like races. Sure there's a bit more to it than in the past, considering time and weather and (hopefully) more advanced AI, but it isn't much more, and with a bigger team that difference should be accounted for and more. Expecting an event count as large as GT4, if not larger, is entirely reasonable. It's not a car model with countless polygons and every tiny detail modeled as accurately as possible. It's a race. They already have rubberbanding AI to make balancing easier. Weather can easily be pre-set, partially randomized, or entirely random. The hardest part is surely choosing how far the cars should be spread apart, which is a problem of their own bad design.
If we assume it is one person making the race events, and we get an average of maybe 4 per month (some months 6, some 3, some 0, etc), that's 1 per week. Assuming a 40 hour work week, which is probably not the case because game developers are often asked to put in extra hours, that's still 40 hours per event. It cannot possibly take that long.
The rate they're added to the game is what would be expected of a side project of one developer on the team.