Coconut (Siamease variety) is a fairly small size coconut variety, as each fruit only has about 200-300 ml of juice. Although the juice is lesser, it is sweet. Typically, the older the coconut, the sweeter it becomes. Siamese coconut, when opened, has a spongy white, thin copra inside and a sweet taste.
When slightly flicking on the coconut with your hand, if you hear a sound then the rice is thick and the juice will be sweet. If you hear a bass sound, the coconut is still young so the coconut copra is still thin, and the juice will not be sweet enough.