Melon pan is a cookie glued to a bread roll, and the calories are exactly what that sounds like. The bun underneath is a normal sweet roll, 60g and 170 calories. The crust is cookie dough made from butter, sugar and flour, 40g and 190 calories, and then a spoon of granulated sugar goes on top for another 30. A konbini melon pan with a 'buttery' crust is 400 to 450 calories, and the big bakery ones at 130g clear 500.
It has almost no protein, about 8g, and 66g of carbs, most of them sugar and white flour. Eaten as breakfast with a sweet coffee it is a 550 calorie start to the day that leaves you hungry by 10am. This is a common Japanese breakfast, a melon pan and a can of cafe au lait from the same konbini, and it is roughly the calories of a full teishoku with a fraction of the staying power.
The filled versions make it worse. A whipped cream melon pan, split and stuffed, is 520 to 580 calories. Chocolate chip crust adds 40. The 'ice cream melon pan' sold at tourist spots in Asakusa and Kobe is a warm bun with a scoop inside for 600 to 700. Compared with all of that, a plain small melon pan at 350 is the sensible one, if there is such a thing.