As slimes and their Nether counterparts are a rarer spawn than other monsters, sometimes their smaller size variants can be an unrewarding, unsatisfying kill when they are less likely to drop slimeballs and magma cream.
The solution to this, could be having slimes/magma cubes grow into their larger sizes! This could happen at a much slower rate than the growth of animal mobs, so that it's not too easy for the players. Also, the growth could only be capable of small and medium kinds, so that the large ones don't grow into giant, OP/unkillable slimes.
Another alternative could be the player feeding (right-clicking or even throwing) the mob. They could be fed with:
- Cocoa Beans
- Nether Wart
Feel free to comment with any other items you feel may be more reasonable food for the slime and magma cube, as well as other arguments/feedback.