okay, why is it that the file sizes for 720p for most episodes length around 25 minutes are now over 500 MiB?

since one week or so the size of those episodes seems to have doubled.

Seem like it been like that for a while even before that season, my guess is that they moved to new encode settings when nobody noticed and assumed that since most people these days have at least 1tb SSD, nobody will cry over it, But I does cause at the end of the day it come to how much animus you can store and I know for fact that for most shows we can still reach to even 350 mb file size if you encode them right

Yeah looks like crunchy switched from CRF15 VBV encodes to 2-pass. I think the new settings are a bit higher quality though (higher maxrate) so there is that.

Yeah, new encodes are considerably better.