AVI file is created by Microsoft and commonly used in video edit, old movies, TV shows, etc. But MP4 is
more popular than AVI and can be compatible on more devices.
Generally speaking, AVI file size is smaller than
MP4. But to be exact, a video size is closely related to the resolution, bitrate, and quality.
Pro tip: If you have successfully change AVI to MP4 yet still think it too large in size, then you could reduce video size on Video Compressor page.