Free storage suddenly dropped while sending file through NFC

[GauravD]

I was trying to send a 200mb pdf file using nfc when I got a storage warning. My free storage had dropped from about 1 GB to 20 MB. I then went and cleared temporary data which increased free space to about 450MB (as other storage dropped to 200MB from 1.4GB). However the System part in Storage Sense now shows 4.7 GB occupied which is way more than what it should be. What should I do to increase free space back to 1GB from 450MB? I use a Lumia 640 XL having Windows phone 8.1 with Denim.