This is one of the few Hong Kong street snacks that is not fried, and it shows. At 130 calories a piece, a put chai ko is a third of a sheet of egg waffle (390) and less than half a fish siu mai skewer (294). There is no oil in the recipe at all; the 1g of fat comes from the red beans. What you are eating is rice starch and sugar set with steam, which is why it is chewy and translucent.
The sugar is the whole story. CFS puts it at 13g per 100g, so one piece is about 3 teaspoons, and the brown sugar version (黃糖) has slightly more than the white. That is a fast hit of carbs with no protein or fibre to slow it down, which is why dietitians in Hong Kong flag it for people managing blood sugar. Two pieces is 26g of sugar, over half the WHO daily suggestion of 50g, for a snack that takes 3 minutes to eat.
Size is the variable. The old style bowl cakes from the Sham Shui Po stalls are about 100g and land at the 130 figure. The mini versions sold in packs of 6 at Taipo and Yuen Long markets are 40 to 50g each, so around 60 calories a piece, which is the number the Centre for Health Protection used in its snack post. Check which one you are holding before you assume it is a light snack.