After digging into the subject, I've found that 1/2 roasted chicken isn't necessarily too much for a keto lunch. The keto diet emphasizes high fat and low carb intake, and chicken, especially its skin, is a good source of this. However, portion control is key and it depends on your personal daily calorie intake. Also, to balance it out, it's good to accompany your chicken with low-carb veggies. It seems that it's not about the quantity, but the overall balance of your meal.