SFH3310 Phototransistor read ADC value
You can use a load line to better understand what is happening. A circuit like this will force the transistor voltage-current relationship to be on a straight line (a load-line). The Y intersection is 3.3V/120000 = 27.5 uA for your resistor choice. The X intersection is your Vcc. Note that the graph is showing the opto-transistor voltage (Vce), your ADC voltage is 3.3 - Vce.

Unless your circuit is more exotic, the resistor choice is a compromise. If the resistance is too high, then high light levels will be difficult to differentiate. If the resistance is too low, then low light levels will be difficult to differentiate.

Your resistance is on the high side, so as you can see in the graph, 100 lux and 200 lux will put out almost the same voltage. (this is not necessarily bad)

If you are looking for a light threshold, you would normally want that threshold to correspond to about half of Vcc. This part has a fairly wide variation from part-to-part, so it might be best to adjust with a pot.

If you are trying to measure the light level, you probably should use a different sensor and a more complicated circuit.