"I am very tall."
Translation:Soy muy alta.
I'm not entirely certain about this, but I think the diffeeence is that "muy" is an adverb and "mucho" is an adjective; while the latter modifies nouns, the former modifies verbs, adjectives, and other adverbs. In this case, "tall" is an adjective, as it modifies a noun. In order to modify it, one would need an adverb; in this case, "muy".