The rice is the biggest number on the plate, and the Padang restaurant knows it. A normal scoop is 250g, about 325 calories, and the free kuah gulai they ladle over it adds another 80 or so because it is coconut milk and oil. Before you even touch the rendang you are at 400 calories. One piece of rendang, usually around 100g with a bit of the dark paste, is about 200 more.
Where people get into trouble is the pick-and-mix. Order at the counter and you take one lauk, that is 690. Sit down and let them stack 12 plates on your table, and you eat the rendang, half a piece of ayam pop, a spoon of dendeng balado and some perkedel. That plate is now 1,100 to 1,300 calories, and it happens without you deciding anything.
The cassava leaves are basically free at 60 calories for a decent handful, and sambal ijo is only about 25 calories per spoon. Those two are the parts you can pile on. The kerupuk jangek (fried cow skin cracker) on the table is not free, one big piece is around 90 calories and it goes down in three bites.
Price check: a nasi padang with rendang is Rp 20,000 to 35,000 in Jakarta, so it is one of the better value meals for the calories if you keep it to one lauk. The protein is decent at 30g. The problem is only when you treat the table like a buffet.