Answer: In SI engines the air-fuel mixture is compressed during compression stroke whereas in CI engines only air is compressed.
The compression ratio is limited in SI engines because if it is made really high there is the possibility of autoignition of air-fuel mixture. On the other hand in diesel engines, only air is compressed hence the possibility of autoignition is eliminated.
Therefore diesel engines can operate at much higher compression ratios typically between 12 and 24 compared to SI engines which operate below a compression ratio of 12