@Jo21 In general that is true however...FFX2 sucked compared to FFX, both KH games were solid to me and is a good example of how 2nd series tend to sell less. GT3 to 4 makes a lot of sense as to why sales wouldn't be as big. GT3 was more arcade style and attracted a larger crowd, GT4 actually employed a small amount of physics and understeer and is closest to a simulation the series has ever gone.
However the Halo series got bigger according the VG data, we can see Halo actually grew in sales. I wonder if the same could happen with Gears.