What is the logic behind the expansion of red blocks?

I think it's dependent on the angle/side you look at the base block.

If you place the initial red block on the floor in front of you and look at it from the above (looking at it's top side), then using the red gun expansion will cause the block to expand into a "floor".

If, in the above scenario, you look at the side of a block (move away a bit and point), then you will create a wall.

Actually, it's not how you look at it but where you put it first. If you start on a wall it will grow on a wall, if you start on the ground it will grow on the ground. Thus, when there is no ground, it grows on the wall. If you want to fill a gap, start from the nearest floor.