Perhaps RAW would not allow it, but I suspect RAI would allow it. If we think about what that 500lb limit represents, it is basically short-hand for :you could lift something about twice as heavy as you." Most medium sized characters are in the 90lb - 300lb weight range. So that would put it on average around twice the weight of your average medium sized creature.
So if we follow that line of reasoning, the spell as cast by a larger creature, such as a Frost Giant, would likely still want it to function as it was intended to be used, which is to lift you to somewhere you cannot reach (I know, a rarity for a Storm Giant, but I'm sure in their Ice Castles they have those cupboards that give them issues). And I imagine if a Storm Giant casts levitate, they can lift themselves as well as anyone, and would probably have a far longer range of about 3-4 body lengths (as 20ft works out to about 3-4 body lengths for the average medium sized creature).
Since no players are ever going to be larger than Medium sized per the precedent that WotC seems to be keeping with, there would not need to be any rules more complex than a 500lb weight limit since that will encompass every possible player character. You don't need to calculate the distance in body lengths based on the caster's height because 20ft works as a good shorthand.
At least, that's how I see it.