You have it wired up wrong. One end needs to go to 5V, the other end to ground and the middle or junction between the two resistors to your analogue input.
Given that the current flows from 5V to the two resistors I would have expected a decrease in the voltage thus reading something smaller than 1023.
No physics doesn't work like that. voltage is developed across a resistor that has current flowing. In your set up you have no (or virtually no) current flowing through the resistors because the analogue input is very high impedance and so no current flows to ground.