Here are some key observations about floor tile.
All floor tile is brittle
When it is applied in a normal manner to a slab-on-ground foundation it is very likely to fracture due to foundation distortion sometime during the life of the house.
Most floor tile is made from clay
Like the clay soils that shrink and swell as it loses and absorbs moisture, clay tiles do the same.
A cracked single tile and two or more adjacent tiles
A single tile that is cracked may or may not be due to foundation distortion. If a crack goes across one tile to an adjacent tile and is on the ground floor, the cause is most likely foundation distortion.
A separation in the grout
It is common to see a separation in the grout between floor tiles. The grout used is a cementitious material and like all cementitious materials, it shrinks over time and develops a visible separation.
Cracks in upstairs floor tiles
These cracks are almost never caused by foundation distortion. The typical cause is that the subfloor is not stiff enough. In an older home that develops floor tile cracks in an upstairs bath, several years after it was constructed, the problem may be water damaged subflooring.
The structural significance of cracked floor tile
Just because you have cracked tiles that are probably cracked due to foundation distortion, that does not mean you have a serious problem. If the contractors would use a high quality thinset and an isolation membrane, the large majority of tile cracks would never develop.