Leche flan is condensed milk you can cut with a spoon. A standard recipe is 10 to 12 egg yolks, a 14 oz can of condensed milk and a 12 oz can of evaporated milk, plus about half a cup of sugar for the caramel. That is about 2,900 calories in the mixing bowl, poured into 2 or 3 llaneras. Kawaling Pinoy's card puts a 1 of 12 portion at 239 calories, and Goldilocks lists their baked flan at 190 per slice, which is a thinner cut.
The yolks are why it hits harder than it looks. Each yolk is about 55 calories and 4.5g of fat, and there are no whites to lighten the mix. Per 100g leche flan is about 265 calories, which is denser than ice cream (about 200 per 100g) and about the same as a slice of cake. A 90g slice does not look like much on the plate, which is how a second slice happens.
At fiestas and buffets it is not really a slice. It is a spoonful on top of halo-halo (about 60 calories for the flan alone), then a real slice later, then a bit more when the tray comes around. Two slices of leche flan is 480 calories, more than the ulam you had for dinner. A slice at a carinderia or bakery is 40 to 60 pesos.