Difference between ffmpeg versions

I am using ffmpeg and ffprobe to do video/audio conversions when using youtube-dl in a GUI interface. The current ffmpeg version command line tool on windows is very large, over 100MB. I want to downgrade it so the installer package won't be too large.

I want to know if there will be any obvious difference, if any, in converted files quality. By eyes and ears I can't spot any and I don't know how to use any tool that measures quality.

EDIT:

The ffmpeg version I want to downgrade to is the one that comes with By Click Downloader. The output when I request the version is below:

ffmpeg version N-70239-g111d79a Copyright (c) 2000-2015 the FFmpeg developers
built with gcc 4.9.2 (GCC)
configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-lzma --enable-decklink --enable-zlib
libavutil      54. 19.100 / 54. 19.100
libavcodec     56. 26.100 / 56. 26.100
libavformat    56. 23.105 / 56. 23.105
libavdevice    56.  4.100 / 56.  4.100
libavfilter     5. 11.101 /  5. 11.101
libswscale      3.  1.101 /  3.  1.101
libswresample   1.  1.100 /  1.  1.100
libpostproc    53.  3.100 / 53.  3.100

That executable is only 28MB.

Edit2:

The video formats I want to use are mp4, webm, flv, 3gp, avi and audio formats are mp3, m4a, wav, aac, flac, opus

Difference between ffmpeg versions

Refer to Changelog. Note that Changelog is not comprehensive; it only lists notable features. FFmpeg development is very active so that means a lot is omitted from Changelog.

If you want to see all changes and updates (there are many) see the git log and refer to each release tag.

It is never recommended to downgrade. You could miss out on thousands of updates and encounter old bugs that have already been fixed.

Smaller ffmpeg

For Windows if you want a smaller executable then try:

The essentials builds instead of full builds from gyan.dev.
Compiling it yourself with help from media-autobuild_suite and choosing only what you need.

Smaller size can mean less available codec and/or formats, depends how a specific ffmpeg version as been compiled you can check on the different versions the available codec and format with: