Ring expansion in cyclo compounds
Simple answer is consider the stability of carbocations. You already got the tertiary carbocation in hand that is much stable than secondary carbocation, which would be resulted by ring expantion. Sure, a methide migration would give you another tertiary carbocation, but before you get that, the reaction progress has to climb another actvation barrier, which would be steep ($3^\circ \rightarrow 2^\circ$). Bottom line is rate of clorination of carbocation is faster than rate of this rearrangement followed by methide migration ($E_\mathrm{a2} \lt E_\mathrm{a3}$):