1/ From an out-of-universe perspective, it's pretty clear that Legolas running over snow was an off-the-cuff addition, designed to make him more 'elvish'. It's not reflected anywhere else in the Legendarium that I can think of. So Tolkien probably didn't think deeply about the details.

2/ From an in-universe perspective, July has the right of it: it's not lightness, it's nimbleness. We know Tolkien had a bit of a thing for tales of American Indians, with their bows and arrows - and their ability to walk through long grass without bending it (and to track by looking for bent grass, a la Aragorn). So Legolas is a super-Native - he's so nimble-footed that he doesn't even make a dent in snow!

2a/ Also, bear in mind it's not like Aragorn and Boromir sank as such. They deliberately carved out the avenues they did, through quite packed snow. If they tried to walk over it, they'd probably be ankle-deep, and in danger of a wrong-foot plunging through a soft patch.

3/ The most fun answer: how much would he have to weigh?

Okay. Snowshoes let you walk on snow. A picture I found online shows a snowshoe with 4.5 x the area of a human foot. Since pressure = force x area, the force exerted by the foot must be reduced to 1/4.5 x the norm by the snowshoe.

Weight is a force. Therefore, to naturally walk like a human in snowshoes, Legolas must weigh roughly 1/4 as much as a human.

For a six-foot human, the ideal body weight is apparently about 170 lb (~80 kg, or 12 stone). A six-foot Legolas would weigh a mere 40 pounds (20 kg, 3 stone). The ideal body weight of a child in kilograms is twice their age plus ten; working this backwards, Legolas would weigh as much as a five-year old.

Since I happen to have a five-year-old, I know that this means Aragorn could pick him up by one arm easily. For that matter, a hobbit could probably manage it. It also means he will never hurt himself by tripping over, unless he actually hits a rock - he's too light to bruise himself much.

He's not in danger of being picked up by the wind (okay, he's got a larger surface area for that mass, but even so), but... yeah, don't let a fighter get in range. Chucking a snowball at him would probably knock him over.

Oh, there's fic in this, for sure...

hS

..not that they weigh so little, so much as that they're so light of foot and graceful.

There are specific ways you can place your stepping to avoid sinking further into things like, say, mud and muck, or through heavy grasses, and also not leave as much an imprint or proof that you had passed through in the first place. Just like if you're trying to step quietly to avoid a squeak in the floorboards or the movement of a loose time, or something, and like there are ways you can walk on a rocking ship and continue in a straight line without ramming into anything or anyone.

I personally consider Tolkien Elves to just have hollow bones, like a bird's. That explains why they are so light. And who said that we couldn't shake elves around like a ragdoll? Elves simply just leap around, never getting into arm's reach while slashing like crazy with their light weapons. That's why we don't see elves with warhammers everyday.

At least, that's what I think. I've never tried fighting an elf before, so I wouldn't know how easily they break.

Can you imagine all the ways that could be exploited?
Tossing a snowball at one would be like launching a cement brick!

Or maybe people could make special elf-proof doors that have walls of snow right behind them, which the Elves can't pass through.

Perhaps the Elves themsElves could exploit it, and make a bridge which is nothing but rope and snow, and everyone else just falls through it.

The possibilities are practically falling from the sky!