If this is the same as the English colloquial "big and strong" you should allow that, even if alto is directly translated to tall.
in another sentence altos means big , and this one means tall, they should fix it
If you would link to the sentence or say what the sentence was that used "big" for "alto" someone might be able to explain the discrepancy.
Because a person cannot be "high" (at least not legally). Only places are high.
"The tall skyscraper" but "the highest point of the plateau/cliff"
Or, figuratively, "the high point of our vacation/adventure"
What if a person is located in a high place? "We are high in the mountains"? Nos estamos altos nas montanhas?