melting point depends on size and most importanatly on packing in the crystal lattice.

increase in melting point is relatively more in moving from an alkane having odd number of carbon atoms to the higher alkane while it is relatively less in moving from an alkane with even number of c-atoms to higher alkane.

reason:

internmolecular forces in a crysatal depend not only on the size of the molecule but also on how well the molecules fit into the crystal latticethe molecules which fit into crystal lattice more easily due to symmetrical structures usually have high lattice energy and therefore high melting points.

c-atoms of carbon chain make a zigzag pattern (c chains are not straight)

in even numbered: terminal c atoms are on same side

in odd numbered : terminal c atoms are on opposite sides

so,even is symmetrical than odd numbered.