"Eye could do that."

yes, before the downfall of Númenor the world was flat. after Ar-Pharazôn, the last king of Númemor, was deceived by Sauron and sailed west to Aman with an army to force the Valar to grant Men immortality (which they do not have the power to do), the Valar ensured this would not happen again by making the world round, so that Aman and Tol Eressëa were "removed from the circles of the world," and Númenor was sunk in the upheaval, ending the Second Age. now when Men try to sail west, they are turned around by the curvature of the world, but Elves can "sail the straight road" and reach the undying lands. some theorize that this means the world is round for Men, but still flat for Elves, allowing them to see beyond the horizon.

however there's not really much evidence in the text for that that I'm aware of, no explicit moment where an Elf sees something that would be beyond the horizon for a Man. there's also a moment where Tom Bombadil refers to a time "before the seas were bent," indicating that the world is round for him, who is neither a Man nor an Elf. and even if this theory were correct, it would not fully explain Legolas's vision; there are multiple instances where he can see something that Aragorn also sees, but in greater detail.

for example, in "The Great River," the second-to-last chapter of Fellowship:

Later as the sun was setting, and the Company was stirring and getting ready to start again, [Aragorn] descried a dark spot against the fading light: a great bird high and far off, now wheeling, now flying on slowly southwards. "What is that, Legolas?" he asked, pointing to the northern sky. "Is it, as I think, an eagle?" "Yes," said Legolas. "It is an eagle, a hunting eagle. I wonder what that forebodes. It is far from the mountains." "We will not start until it is fully dark," said Aragorn.

then in "The Riders of Rohan," the second chapter of The Two Towers:

"Look!" cried Legolas, pointing up into the pale sky above them. "There is the eagle again! He is very high. He seems to be flying now away, from this land back to the North. He is going with great speed. Look!" "No, not even my eyes can see him, my good Legolas," said Aragorn. "He must be far aloft indeed. I wonder what is his errand, if he is the same bird that I have seen before. But look! I can see something nearer at hand and more urgent; there is something moving over the plain!" "Many things," said Legolas. "It is a great company on foot; but I cannot say more, nor see what kind of folk they may be. They are many leagues away: twelve, I guess; but the flatness of the plain is hard to measure."

now obviously the horizon isn't interfering with Aragorn's ability to see a bird in the sky, yet he can't see it while Legolas can. also, for 12 leagues, or 36 miles, to be within the horizon, which it must be for Aragorn to be able to see, they would have to be at least 865 feet above the plain. they are described as standing on a ridge twenty fathoms (120 feet) above a cliff, which falls an unspecified distance to the plain, so that must be at least another 745 feet

then later in the same chapter:

Following with his keen eyes the trail to the river, and then the river back towards the forest, Aragorn saw a shadow on the distant green, a dark swift-moving blur. He cast himself upon the ground and listened again intently. But Legolas stood beside him, shading his bright elven-eyes with his long slender hand, and he saw not a shadow, nor a blur, but the small figures of horsemen, many horsemen, and the glint of morning on the tips of their spears was like the twinkle of minute stars beyond the edge of mortal sight. Far behind them a dark smoke rose in thin curling threads. There was a silence in the empty fields and Gimli could hear the air moving in the grass. "Riders!" cried Aragorn, springing to his feet. "Many riders on swift steeds are coming towards us!" "Yes," said Legolas, "there are one hundred and five. Yellow is their hair, and bright are their spears. Their leader is very tall." Aragorn smiled. "Keen are the eyes of the Elves," he said. "Nay! The riders are little more than five leagues distant," said Legolas.

again, both Aragorn and Legolas can see something, but Legolas sees it in much greater detail. they are described standing on top of a hill, with the edge of Fangorn forest visible at a distance of ten leagues, so the hill must be at least 600 feet tall

Yes. It’s akin to a woman saying something like, “Can you help me lift this with your man arms?”

Though even more so because they are straight-up different species and the difference in vision is supernaturally significant.

Maybe it’s more akin to asking a dog to smell something with their “dog nose”.