Ring Strain : Cycloalkanes

Table 1.1 lists the heat of combustion data for some simple cycloalkanes.  These cycloalkanes do not have the same molecular formula, so the heat of combustion per each CH₂ unit present in each molecule is calculated (the fourth column) to provide a useful comparison.  From the data, cyclopropane and cyclobutane have significantly higher heats of combustion per CH₂, while cyclohexane has the lowest heat of combustion.  This indicates that cyclohexane is more stable than cyclopropane and cyclobutane, and in fact, that cyclohexane has a same relative stability as long chain alkanes that are not cyclic. What causes this difference in stability or the strain in these two small cycloalkanes?