"light is lost or wasted in filling the whole space, true?"

Yes, and no.

Most led emitters have a "lambertian" pattern.
For practical purposes, call it 180°.
(Gas discharge lamps are app. 360°)

In a 4 X 4 room, with a 4 X 4 lamp, servicing the plants is a chore.
And the lamp footprint is calculated for open space.

In a 5 X 5 cabinet, with reflective walls, a 4 X 4 lamp is ideal for the plants, and the grower.
If you try to stuff a 5 X 5 room to the max. it becomes a serious chore as they get bigger anyway.
Just not enough elbow room, or coverage.

I think that "Solar Flare" lamp is lensed to 60°.
Have to go back and look again, but the point is 60° lensing would be tight for those dimensions.
90° would be mo' betta for that area.

Ideal, for me, would be 2. 4x4 arrays in a 5x10 space.
Alas, not an option here.
But, if you are defining your own space . . .

This:
Attachment 297819Attachment 297820

is how I tested for reflectivity and space usage within an enclosed space.

Start with a stable source and distance, then try different reflective surfaces.

(The reflectix won, followed closely by Titanium oxide white paint.)

Aloha,
Weeze