Everyone blames the custard but the pastry is the real story with egg tarts. That flaky, melt in your mouth shell is made with a lot of butter or lard layered into the dough, the same trick that makes croissants so calorie dense. The egg custard inside is mostly egg, milk and sugar, which is moderate. So a single tart lands around 195 calories, with the crust doing most of the damage.
They are small and they come in boxes, that is the danger. One egg tart with your coffee is a fine treat. Four egg tarts because the box was there is 800 calories you did not plan for.