The native camera app records video in the highest possible resolution supported by your device. The output files thus generated are very huge and quickly take up a substantial amount of space on your device.
Video Sizer lets you choose the video resolution, so you can record at various quality levels. The output file size is greatly reduced, thus allowing you more recording time with the same available free space of your phone.
The smaller files have advantage that they can be uploaded to FaceBook and other video sharing sites a lot faster than the default video camera output files.