youre either to fat
or very skinny.
on a serious note, as others have mentioned it is a very common practice to have a set of life jackets installed on the aircraft for all the crews to share.
1. it is easier to keep track of the jackets for service and maintenance
2. The normally don't disappear or run over its maint tx due to being in a locker somewhere with the pilot in question being on 6 weeks leave.
in addition most of the life jackets also have a ELT/PLB installed which is again coded to the aircraft.