In disubstituted cyclohexanes, the bulkier substituent favors an equatorial position due to the steric interactions. methyl, ethyl, and isopropyl groups (when at axial positions) do not increase the ring strain as much as the tert-butyl group on the axial position.

Draw the chair form for the axial conformations of cis-1-isopropyl-3-methylcyclohexane and cis-1-tert-butyl-3-methylcyclohexane to explain why the tert-butyl group causes a large increase in energy compared to the isopropyl group.