As mentioned in another thread, "In fact, tier +2 and above are already cheating... +2 machines use 9× space of original but having 16× speed."

For example, Chem Plant takes 3x3 tiles on the map, and Chen Plant +1 takes 6x6, but Chem Plan +2 is only 9x9, where perhaps it should be 12x12.

Any reason it was decided to size up sub-linearly like this and not scale up to the actual amounts? Any thoughts about an option to have more linear sizing? I guess the recipe to build these larger machines requiring modules kinda explains the space savings; so if we added linear sizing the recipe could also just be straight up 4x the previous size, plus some low-cost items like belts or inserters to make up for the savings from not having to build all the extra belts and insterters if using vanilla setups. :)

Exciting mod, though! It looks like it could also work well with Deadlock Stacked Recipes as another way to scale up. :)

Since start I wish to apply the ideal linear scaling in sizes, so no longer having any "size cheating" issue.
However the game engine limitation prevents that. When an entity have large sizes (say, 40+ tiles), the animations and other things start to become unstable. From my experience in Schall Endgame Evolution, oversized biters (category XX) also have pathfinding issues, which ended up just idling there. They also gave performance hit, as it seems the rendering of large entities kills FPS.
More such explorations give a conclusion that the game engine does not support very large size entities.

Strict linear scaling means that we can only have at most +2 on most machines. For example, an Oil Refinery +3 is already at 40x40, and the game engine does not like +4 and above. (At least it was the case when I create this mod.)
Have only up to +2 in linear scaling is only 16x speed, which does not seem that useful (for my purpose to improve UPS in megabase.)
That was why I have applied non-linear scaling (square root of speed): it keeps a simple relation between size and speed, and lets the high overtier machines work in megabases (instead of killing FPS).