When I activate the limit and it is set to 2 GB, it seems that 12 GB will be saved. Not 600. It is true?
If you set the prune limit to 2 GB, you keep 2 GB of data from the blockchain and the UTXO set which should be around 11 GB in size as of now.
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It's also worth to mention size of UTXO usually grow slowly over time, with exception of Ordinal boom which double it within a year.
And one more question. Does the pruned node do the same thing that full does? I want to know if what I am doing is valuable or not.
Mostly same, but pruned node only share 288 most recent blocks to other nodes, no matter size of recent blocks a prune node store. IIRC this behavior is intended to prevent node fingerprinting.