A PDF, which original is around 500kB, can easily get converted to a (redacted) PDF around 2.5 MB when you redact it.

Also, I have the feeling that multiple redactions increase this effect, i.e. each time you redact, save, and redact again the file size increases.

IMHO, that is both not good, because:

1. you actually remove information (or replace it with simple rectangles/censor chars), so it should theoretically at most be the same file size.
2. given that maybe your PDF converter or things interfere, the size increase should at least not be that drastic/big.

In my case both the original and the redacted version also contain the selectable text, so the argument that you use OCR and this adds information does not count. 😉