This is a whole bird. A young chicken of about 500g, skin on, bones in, gives you roughly 300g of edible meat and skin and comes to 600 calories. The skin is the swing factor: eat it and you get the fat, peel it and you drop about 150 calories while keeping nearly all of the 70g of protein. Nobody at a samgyetang restaurant will judge you for peeling.
The stuffing is the surprise. The 60g of raw glutinous rice packed into the cavity swells into a thick porridge inside the bird and is about 210 calories on its own. Then the restaurant usually brings a side bowl of rice too, which most people do not need. Skip the extra rice and you are already saving 300.
At 15,000 to 20,000 won a pot, samgyetang is a meal, not a soup course. 918 calories with 70g of protein is honestly a fine lunch if it is your main meal of the day. The problem is treating it as a soup and adding rice, a plate of kkakdugi, and a shot of insamju on the side, which pushes a hot summer lunch to 1,200.