The phrase "have a huge appetite" can be said in a variety of different ways. Some synonyms for this term include "ravenous," "insatiable," "voracious," "gluttonous," and "hungry as a bear." These phrases all convey the same meaning of someone who has a strong desire to eat.
Other synonyms for "have a huge appetite" might include "never satisfied," "always hungry," "constantly craving," or "can't get enough." Each of these phrases adds a slightly different nuance to the idea of having a big appetite, but all of them communicate the idea that the person in question is always looking for more food to eat.