The chicken itself is not the issue. Grilled thigh and drumstick with skin is about 220 calories per 100g of edible meat, and a paa gives you around 170g of meat off the bone, so about 375 calories. What pushes inasal past a plain grilled chicken is the atsuete oil. It is chicken fat rendered with annatto seeds, brushed on during grilling and then served in a little dish for your rice. Kawaling Pinoy's recipe card lands at 450 calories per quarter and uses a full pound of chicken skin to make the oil.
The pour matters more than the piece. Each tablespoon of chicken oil over your rice is about 120 calories. Two tablespoons, which is what most people do without thinking, adds 240 calories to a plate that already has 260 calories of rice in it. The pecho (breast quarter) is a bit leaner at around 380 calories, but the difference is smaller than you would guess because the skin comes with it.
At Mang Inasal a paa with rice costs about 140 pesos and the unli rice is the trap. Every refill is another 260 calories. One paa, one cup of rice and a spoon of chicken oil is about 830 calories. One paa with three cups of rice and chicken oil on each is about 1,450. Same order slip, twice the calories.