The only reason I know why an OS would use a lot of data connection is downloading upgrades. Did your phone install (a) lot of system updates recently? Are you using stock firmware or did you install a custom OS like CynogenMod, possibly even a nightly build that updates every night?
For Chrome data usage there may be a lot of reasons - it usually completely depends on what you do with your browser. If you download large files, for example, that would be counted as Chrome data usage, too. Also watching videos or complete movies via browser will be accounted as usage for Chrome.
Maybe you might want to try 3g watchdog, which is a very good app for traffic supervision that I use for 2 years now - perhaps that would split the individual system components using wifi data better.