You're not adding the size of the ffmpeg binary itself with that.

My ffmpeg which I compiled as follows (with my own filter)

  --enable-gpl --enable-version3 --enable-nonfree --enable-libx264 --enable-libx265 --enable-libmp3lame

is 21,387,880 bytes

Running with your command reports an additional 18,273,624 bytes

linux-vdso.so.1 (0x00007ffcbcdfd000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007ff60391b000) libmp3lame.so.0 => /usr/lib/x86_64-linux-gnu/libmp3lame.so.0 (0x00007ff6036a4000) libx264.so.152 => /usr/lib/x86_64-linux-gnu/libx264.so.152 (0x00007ff6032ff000) libx265.so.146 => /usr/lib/x86_64-linux-gnu/libx265.so.146 (0x00007ff60267e000) libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007ff60245f000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff60206e000) /lib64/ld-linux-x86-64.so.2 (0x00007ff605d14000) libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007ff601e6a000) libnuma.so.1 => /usr/lib/x86_64-linux-gnu/libnuma.so.1 (0x00007ff601c5f000) libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007ff6018d6000) libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007ff6016be000)

I guess it depends what you compile in.