General opinion is that a side job is one that supports the main income of an individual, often times perceived as something that brings little money to add to what you make from your day job. If Mr X earns better from a side job when compared to the income from his career option, does the side job still qualify to be called a side job?
I'm of the opinion that the "main job" is one that you are formally employed in, regardless of how much of your total income it contribute. So if you're an in-house graphic designer and doing a side hustle of doing commissions online, then your main job is with the design company, whether it makes more than your side-hustles or not.
If you do get to the point where your side job is earning you more, then you might have to start considering whether you should be making more time for it and whether to do it full time. Some people are under this situation but decided to keep the main job for certain benefits like insurance for example.