When a C atom forms a bond with another C atom , then the bond formed is a sigma bond. And we all know that if a bond is formed then energy is released. And stronger the bond ..it will be more stable so more energy will be released.
Now when the C atom forms the double bond then a pie bond is formed. Similarly for a triple bond also a pie bond is formed. And we read that a pie bond is weaker than a sigma bond because of less overlapping between the orbitals. So less energy will be released than the first bond.
