The sanded grout is ideal for joints with a minimum width of 1/8″. For smaller joints, you should use unsanded grout. Sanded grout is bulkier and has large sand particles. Using it on joints smaller than 1/8″ can make them crack.
You’ll also have to use more water to make a thinner consistency. This water evaporates over a long time, leaving air spaces. Grout above these spaces collapse, thus forming pinholes.