A side job IMO is not really meaning that it is a lesser paying job from one originally known with. The main job is usually the one everyone knows you are doing but the side job is known by few, maybe close colleagues. It is more of diversification to me but whether called side or not doesn't matter and these days it has been the one more sustaining because it gives you time and you still earn from it. The main job is the one taking most of your time and might pay you little .
So I think what is called as side job is not really a side job but another job in diversification. When we diversify, we say is side job but sometimes proceeds from it is not small, it can be higher or measure up to main job. There are many main job considered as side jobs for others and some side jobs known for others as main jobs.