Yes, you can have too much light. Some plants don't like the high watt lights, and will wither away, and there is a limit to how much light the plants can utilize. If the nutrient level is too low, or there isn't enough CO2 available, all the light in the world won't help the plants. And, if this is the situation, you often end up with problems--algaes, browning leaves, etc.
So, 110 watts on a 20 might be too much, or it might be perfect--it depends on other factors. I am running 120 on my 40 right now, and about 100 on a 55. Plants in both tanks are doing well, with minimal fertilization (good substrate for both, though) and a bit of CO2 supplementation.