When I spawned a few slimes of different sizes and quit then re-enter the world, the slimes' sizes has gone smaller, i.e.: Big Slimes -> A slime size between big and small with 10 HP and 5 hearts (Bugged slime); Small slimes -> Tiny Slimes; Tiny Slimes -> Despawn
If you did not already, please remember to perform a search before creating a ticket to see if your bug hasn't already been reported.
This issue was reported previously as MCPE-7972; comment your experience there!