Riddle me this...

[TehCthulhu]

A guy brushes his teeth and goes to bed. He wakes up the next morning, and everything seems the same. He goes into the bathroom to brush his teeth again, turns on the sink, and realizes he is in a very different place. How does he know?

Water goes the wrong way down the sink.

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?