The grout can develop cracks owing to a number of reasons. Joint Size and Inadequate Grout Filling
Joint size plays the determinant factor in selecting the type of grout required to fill the gaps between the tiles. If the tiles size is too wide say more than 1/8 inches then it have to be filled with sanded grout. The sand presence in the grout ensures that the grouting layer does not shrink and produce wide gaps or cracks after drying.