This means for a 30 day retention window your barman volume needs to be over 30× the size of your postgres volumes.
I only think this would be true if you were at capacity on your postgres volume. Although the barman volume needs to be at least as large as your postgres volume, each backup only seems to use the storage necessary for the data being backed up (which could be quite small). That’s what it seems to do when I do a barman backup but maybe I’m missing something though!