But ffmpeg wasn't installed before, x264 was but I can't tell as a dependency to what. It is x264 update that brings everything in:
pacman -S x264
world/ffmpeg 1:4.1.2-2 29.13 MiB
world/jack 0.125.0-7 2.83 MiB
world/lame 3.100-2 0.97 MiB
world/libiec61883 1.2.0-5 0.15 MiB
world/libvpx 1.8.0-1 4.76 MiB
world/opus 1.3-1 1.77 MiB
world/sdl2 2.0.9-1 2.94 MiB
system/sysfsutils 2.1.0-10 0.15 MiB
world/v4l-utils 1.16.3-1 4.16 MiB
world/zita-resampler 1.6.2-1 0.24 MiB
world/x264 2:155.r0a84d986-2.1 2:157.r72db4377-1 0.08 MiB
# pactree -r x264
x264
└─gst-plugins-ugly
# pactree -r gst-plugins-ugly
gst-ugly returns nothing, like it is an orphan
trying to remove it I get this:pacman -Rnsu gst-plugins-ugly
a52dec 0.7.4-10 -0.15 MiB
l-smash 2.14.5-1 -1.04 MiB
libcddb 1.3.2-5 -0.16 MiB
libcdio 2.0.0-1 -0.88 MiB
libdvdread 6.0.1-1 -0.31 MiB
libmpeg2 0.5.1-6 -0.25 MiB
libsidplay 1.36.59-9 -0.28 MiB
opencore-amr 0.1.5-2 -0.27 MiB
x264 2:155.r0a84d986-2.1 -3.70 MiB
gst-plugins-ugly 1.14.4-1 -1.28 MiB
In other words it seems as if I intentionally had in the past installed gst...ugly, which needed x264, and new x264 needs much more than it did in the past. It may be the case that I had installed mpv/smplayer and removed it with simple pacman -R and it left its trash behind.