Compress multiple videos at once on a Mac
Handbrake can queue up a batch of files.
FFmpeg is free and works on a mac, but you'd have to run it from Terminal.
You can install it using Homebrew for Mac.
Once Homebrew is installed, go to Termaninal and type
brew install ffmpeg

Then you should be able to change directory to wherever your files are by typing something like:
cd /Users/my.username/Movies

where my.username would be your username on that mac, and Movies would be whatever directory the files were in
Then you can convert them by typing something like
ffmpeg -i input.mp4 output.webm

Where input.mp4 is your source file, and output.webm is your output file.
To convert all the mp4 files in that directory you would enter
ffmpeg -i *.mp4 output.webm

More info on selecting different codecs etc.