How much CO2 is produced in burning of a candle?
Surely $\ce{CO2}$ will fill some of the space of used up $\ce{O2}$. Candles these days are commonly made out of paraffine, which is basically $\mathrm{C_{many}H_{many}}$. The equation may look as follows:
$$\ce{C31H64 + 47O2 -&gt; 31CO2 + 32H2O}$$
You see that we get approximately 2 moles of $\ce{CO2}$ for 3 moles of spent $\ce{O2}$, hence the decrease in volume.

Another factor is that $\ce{CO2}$ is much better soluble in water than other components of air, so over time (though not immediately) it will probably vanish from the glass and thus free some more space.

Yet another factor is that air in the glass is quite hot while the candle is burning. As soon as it cools down, it shrinks.