A
software synth can be a few megabytes and downloads virtually instantly. Your computer calculates or generates all the sounds.
Software synthesis can even recreate acoustic instruments with increasing accuracy.
Pianoteq 6 is an example of such as synthesizer:
https://www.pianoteq.com/instrument_list
Obviously there are many software synths recreating hardware synthesizer sounds and new synthesis methods.
Sample libraries such as Kontakt eat up gigabytes of storage for their recorded sounds. As a general rule they are best used for replication of various acoustic instruments. Those acoustic libraries can be huge and expensive.
The price of sample libraries vary wildly. The most expensive soft synths are relative chicken feed compared to the most expensive sample libraries (mainly aimed at orchestral mock ups).