In double and triple bonds, more electron density piles up in the region between the nuclei, so the nuclei are drawn together even more closely by their attraction to the electron density. However, π bonds don't build up electron density directly between the nuclei, so π bonds don't add as much to the bond strength as σ bonds which do. π bonds shorten the bonds too, but their shortening effect is not as great as the effect of the σ bonds. Of course, π bonds always add to the effect of the σ bonds and therefore double bonds (σ + one π) and triple bonds (σ + two π) are stronger and shorter than single σ bonds alone.
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