The only way to effectively shrink the PS is to close (consolidate) entire programmes - something that is very, very hard to do because every single one has a "cheering section" of voters - and let attrition do its thing.
A good, effective public service needs constant renewal AND it needs good, experienced senior management; hence "natural" attrition.
One way to force some useful attrition is to get rid of some minor portfolios and make several serving ministers into associate (junior) ministers in other departments and then reduce the number of DMs, Assoc DMs and ADMs with their attendant (and generally late large) bands of acolytes.