The Android 4.4.3 KitKat update for Nexus 7 is about 73.9MB in size and brings performance improvements, stability and bug fixes. Notably, the Android 4.4.3 KitKat update is an incremental update from Android 4.4.2 KitKat and brings security enhancements and various bug fixes.
It is a small upgrade, so most changes on it is bug fixes.
The unofficial Android 4.4.3 changelog follows below:
frequent data connection dropout fix
mm-qcamera-daemon crash and optimization fixes
camera focus in regular and HDR modes fixes
Power Manager display wakelock fix
multiple Bluetooth fixes
fix for a random reboot
app shortcuts sometimes got removed from launcher after update
USB debugging security fix
app shortcuts security fix
Wi-Fi auto-connect fix
other camera fixes
MMS, Email/Exchange, Calendar, People/Dialer/Contacts, DSP, IPv6, VPN fixes