The skin is the whole story. Lechon skin is rendered pork rind, essentially chicharon that never left the pig, and it runs about 500 calories per 100g. The meat underneath, once the fat cap is trimmed, is about 220 calories per 100g. So a 120g plate with 30g of skin and 90g of meat is 380 calories, but the same weight of pure skin would be 600. At a fiesta, the people fighting over the crispy bits are eating twice the calories of the ones taking meat from the loin.
Lechon houses sell by the kilo, and a quarter kilo (250g) is a normal single order for one hungry person. That is about 800 calories before rice and before the Mang Tomas. The liver sauce is 40 calories per 30g spoon, which sounds small until you use four spoons. Cebu lechon skips the sauce entirely because it is stuffed with lemongrass and salt, so the count is a bit lower per plate.
The morning-after version matters too. Leftover lechon becomes lechon paksiw (simmered in vinegar and liver sauce) at about 350 calories per cup, or gets fried into lechon kawali style pieces at 500 plus. Most fiesta lechon is eaten three times over two days, and the 380 figure is only the first plate.