Ukraine has S-300 systems already.

This is an additional system, the systems are fairly massive(Consisting of Radar, Control modules, several/dozens of launchers each). S-300 systems are not small, but Ukraine does have up to 100 batteries and 300 launchers with 21 confirmed launchers destroyed.

What greece would be donating is 32 launchers, roughly 4 systems.

Now keep in mind we only know Ukraine lost 21 launchers, likely they have lost more, and generally it was predicted only about 40% of them even worked prior to 2022. Ukraine has ramped up repair and reservice of the systems but that could take over a year or longer to bring online older systems or salvage usable systems.

Either way, 32 additional launchers and 4 additional systems is good news if ukraine gets them. It's just more to cover more area so less gets through.