In sigma bonds, linear overlapping occurs whereas in pi bonds, parallel overlapping occurs. In pi bonds, the electron density is parallely concentrated, which results in less stronger bonds. In sigma bonds, electron density is concentrated along the x-axis.
The greater the extent of overlapping, the higher the probability of finding the valence electrons in between the nuclei, making the bond stronger and shorter. This explains why sigma bonds are stronger than pi bonds!
Below is a link of the comparison of the two bonds:https://i.stack.imgur.com/g7mqW.jpg