Riddle me this...

[TehCthulhu]

Where the hell did you get carbon dioxide? It's darkness.

And both options yield the same amount, 2 * infinity = infinity

I belong in a line with all others like me, some greater, some lesser. When I get put next to you, you get smaller. When I stand under you, you get bigger. When I stand under you when I'm upside-down, you get smaller. All these values are absolute.

What am I?