Red bean ice tops every cha chaan teng drink ranking in Hong Kong. CFS measured it at 87 calories per 100ml, which is more than a cola (about 42). The reason is that the red beans are cooked in sugar before they go in, then the drink gets syrup, then evaporated milk on top. There are three separate sugar sources in one glass. A 300ml glass ends up around 260 calories, equal to a bowl of rice.
The evaporated milk is the only thing giving you any nutrition. About 50ml adds 70 calories, 4g of protein and most of the 5g of fat. The beans themselves have some fibre, but 100g of sweetened red bean paste is about 150 calories, and that is what the bottom third of the glass is. So a drink that looks like a snack is really dessert.
Less sweet barely helps here. CFS found the less-sweet version still lands at 240 calories and 30g sugar, because the beans are pre-sweetened and the shop cannot take that out. This is one drink where the 少甜 order does very little. If you want it, have it as your dessert, not as your drink with lunch.