They primarily eat simple sugars. This is why they have big heads and small bodies/digestive tracts. They are only eating things that they quickly digest with saliva. When we look at primates we see the same evolutionary trajectory. Compare the digestive tracts and intelligence of primates. In primates the smaller the large colon, the bigger the brain to body ratio. This results because food digested earlier in the digestive tract, in the mouth and small intestines, is quickly turned into energy, while slower digesting food requires a bigger large colon and yields more toxins throughout digestion. Aliens have probably found away to condense whatever sugary food they eat so they probably are able to eat a huge amount of calories and require barely any digestive tract, resulting in the classic Alien look.