Here are the view points from different sides. Size does matter

1. factory side
Bigger is easier to build, more sales revenue.

Labor cost of builiding smaller one is actually equal or even higher because the margin for errors is also smaller and requires more skillful workers who are harder to find or to keep in places like China.

Economy of scale is small. the facotries have to build limited quanity and there is no way that they can keep running if they can only a few hundred a year while the foamies can be sold for several thousands per year. The unit cost is high, so they will lean toward whatever size that can command higher revenue.

2. pilot side

Bigger is easier to see, easier to fly, and looks cooler and higher value. Now the foamies are approaching the size 60-100N turbine jets and they cost around 200-500 bucks. It would be hard to tell your wife that your composite jet is 10 times more than that same size foamie jet sitting in your garage.

I think the optimal size is about 120-160s. It's the medium size now. It's like our body size. Our ancestors average smaller than we are now. It's just a progression through evolutions.

My every day jet is Viper 1.9M because it's small but easy to fly and maintain.

My Wren 44 size A7 has been sitting in the garagee for 5 years and I won't even want to fly it because my reflect and eye sights are going downhill every year. It costed me $6000 RTF, while I can get a big 120-160 size jet for the same price or less.
Mike