Cracks in grout

Hi Guys,

I need some advice please. More often than not my grout cracks when it dries. I follow the packet instructions to the T and I've tried different brands of grout. Sometimes the cracks get really big, but mostly its hairline cracks. This has happens on plastic, glass and wood surfaces.

If the gaps between tiles are really small, then it doesn’t happen, but I can’t always have small gaps between tiles. What am I doing wrong?