Convert .WMV using FFmpeg
wmv is often detected as 90k FPS by ffmpeg.  (That's a placeholder value since the demuxer doesn't look into the stream right away, to see the intervals between the first few frames).

ffmpeg's default for mp4 output is CFR mode, hard-duplicating frames to bring the framerate up (to 90000FPS in this case).  Even the most efficient codecs take a few bytes to code a frame that's an exact duplicate of the one before.

-vsync vfr (same as -vsync 2) on the command line should fix the problem (by letting ffmpeg not drop or duplicate any frames, but instead just use the input timestamps for the output frames).  It goes after the input file but before the output file, as it's an output option.

Other than that, see https://trac.ffmpeg.org/wiki/Encode/H.264 for example for how to choose settings appropriate for what you're doing.  (e.g. -preset veryslow to spend lots of CPU time for an encode-once task where the quality-per-filesize gain will benefit lots of times.)