Thing is, sci-fi doesn't have to be a futuristic setting. My comic, City of Blank, is included in those two tiny pages of sci-fi comics on here, and if anything, it's behind our society in technology (It's about on par with our world's technology of the 80's). I think what separates it and makes it a smaller genre is the fact that everything has to lie within logic and explanation. Nothing can just rely on "because magic" in the sci-fi genre. I have purple, ghostly, face-stealing monsters in my comic, but there's nothing "magic" about them. I go into explanation of how they work, what they're made up of, and how they develop, grow, and reproduce.
When I first did join Tapastic, I actually DID categorize my own comic in "fantasy" because...purple, face stealing ghosts! But a couple of people on here actually recommended my comic as a sci-fi and after that I started to realize it really does fit better into that category, because there's nothing "magical" about the mysterious things in my comic. Everything has a scientific explanation that makes sense in that world, and for the most part, our's as well.
Point I'm getting at is... I think sci-fi is a smaller category because in a way it is...a little harder to work with? I mean, to be fair, no GOOD fantasy uses "because it's magic!" to explain the logic of their story. Harry Potter goes into heaps of detail of how things in it's magical world works with nothing being as simple as "because magic." But in the end, I think making a good sci-fi is a bit harder than making an average fantasy and that might be why it's more limited.
By the way, welcome to Tapastic! You'r comic looks awesome and I definitely plan on checking it out when I have time. The alligator/crocodile people in the image alone sold me. :'D