Recently I had reset my windows phone. At that instant OS took 1.92 GB but now it is about 2.30 GB. Can somebody explain to me what made OS take more memory?
If you're talking about just the reserved data for the OS, there are a number of things that use this up. One of the big ones is updates; updates themselves may make the OS bigger, but also downloading the updates uses up space. Stuff like temporary Internet files and the like also take up some space, though I'm not sure what category they're reported under in Storage Sense.
Also, if the 1.92GB was immediately after resetting, before you set up your Microsoft account again (or at least before it restored the backup from that account), then that might include data which is included in the system files category, stuff related to your account details, maybe even things like your backed up SMS and such.