I dont know what your definition of a sci-fi game would even mean.
A sci-fi game tends to treat the physical reality of the world as more-or-less obtaining, but then tweaks things with speculative technological developments, permitting FTL travel, or something similar.
But in a sci-fi game we are generally expected to think of biological things as having evolved through some combination of natural selection and biochemical processes; to think of stars and solar systems as having formed thorugh some combination of nuclear and gravitational processes; etc.
In my Classic Traveller game, the PCs have used medical laboratories to undertake biochemical examination of various living things from an alien world, to try and identify which ones evolved on that world and which ones came from elsewhere. (Traveller posits a world with a fairly high degree of interstellar colonisation by humans and other sentient beings.) That depends upon an assumption that the fictional world - although it is, in fact, most likely physically impossible because including FTL travel via "jump" drives and "gravitic field" generators - correlates to a high degree to the scientific reality of our world. There is also a broader underlying assumption that the world is a mechanistic one amenable to natural explanation via scientific means.
None of these assumptions is true of the D&D world. Lifeforms in D&D are created, not evolved. The elements are air, earth, fire and water - plus (in some schemes) positive (life) and negative (death) energy. The performance of magical spells is commonplace.
This world seems obviously not amenable to any sort of explanation of the sort that would be recongisably scientific. It is not a mechanistic world; not a world in which the best explanations are naturalistic; not a world of processes amenable to mathematical modelling.
So are you saying that imaginary creatures do not have to be the same as humans? That sounds more like my definition of Giant as opposed to your claim that Giants could not even support their own weight.
I'm saying that once we start talking about human-like beings who can nevertheless support their own weight, we're talking about a world which has very different biomecanical proceses from our own. And once we toss in
magic, there's no point in even trying to conceive of it as having
natural processes in the
scientific sense at all.
It does make sense to view D&D as having such scientific categories as gravity and fluid mechanics. It just ALSO has magic, which when applied to the former, supersedes them. The point you two are making not logical. If the world didn't have gravity like we know it, dragons wouldn't NEED that magic to fly.
Gravity as we know it - or, at least, as
I know it - is a univeral force that all masses exert upon all other masses. (Yes, my knowledge of gravity reflects Newtonian conceptions - my general relativity is weak to the point of non-existence. But I'll plough on.)
Aristotle, and Aristotle's toga-maker, both knew that dropped or otherwise unsupported objects fall to the ground, and that bats and birds need to flap their wings to take off. But they didn't think the world had gravity as I know it. The idea of universal gravitation was still about 2000 years ahead of them.
In other words, envisaging a world in which dragons need to flap their wings to fly is not the same thing as envisaging a world in which gravitation as I know it operates. And given that the only treatment of planetary motion in an official D&D sourcde that I'm aware of is Spelljammer, and it's account of planetary motion has nothing to do with gravity at all, there is good reason to think that there is no universal gravitation in the D&D world.
What happens in the D&D world if a person tries to measure the density of their world by means of a torsion balance (a la Cavendish)? I think the rulebooks leave this a completely open question - or, rather, they assume that this won't happen.
Just the same as, in Traveller, the game just assumes that no one will actually ask what a jump drive is or how one works, because (of course) there is no coherent answer to that.
In any event, to return to the logic of the point that I am making (and that [MENTION=205]TwoSix[/MENTION] has made):
It does make sense to view D&D as having such scientific categories as gravity and fluid mechanics. It just ALSO has magic, which when applied to the former, supersedes them.
It's of the nature of scientific categories that they describe natural phenomena or natural processes that are not arbitrarily "superseded". That's what makes it science.
Consider this, from p 58 of Gygax's DMG, under the heading "Travel in the Known Planes of Existence":
uppose that you decide that there is a breathable atmosphere which extends from the earth to the moon, and that any winged steed capable of flying fast and far can carry its rider to that orb. Furthermore, once beyond the normal limits of earth's atmosphere, gravity and resistance are such that speed increases dramatically, and the whole journey will take but a few days. You must then decide what will be encountered during the course of the trip - perhaps a few new creatures in addition to the standard ones which you deem likely to be between earth and moon.
Then comes what conditions will be like upon Luna, and what will be found there, why, and so on. Perhaps here is where you place the gateways to yet other worlds. In short, you devise the whole schema just as you did the campaign, beginning from the dungeon and environs outward into the broad world - in this case the universe, and then the multiverse.
The foundations of that are Edgar Rice Burroughs and co; not Newton, Einstein and Maxwell! There are tropes, but there is no default assumption that the world is one that resemble the real world in respect of gravity or fluid mechanics (given that he is positing that a flying horse can fly from the earth to the moon!).
That seems to me to be the whole point of the fantasy genre.