scour VS optipng-bin

You could install the optimizer Inkscape uses internally and process your files with a super simple shell script. When reading the inkscape command line help, it does not seem to offer any option to export optimized SVGs from there

Inkscape uses Scour under the hood for optimized SVG export.

First make sure all your icons are saved as 'Optimized SVG' to remove Inkscape specific data and unnecessary id-attributes. Inkscape uses 'Scour' for this under the hood, so you can just use that directly to convert your files from the command line. I recommend the options --strip-xml-prolog --remove-metadata --enable-id-stripping --renderer-workaround. Now you only need to replace the outermost ... with ... for each file and remove all attributes on it, except width, height viewBox and preserveAspectRatio. Since we've stripped the XML-declaration with Scour, simply concatenate all files, wrap the result in a new element and lastly give your symbols unique ids and make sure all remaining referenced ids are unique between symbols.

The script optimised the SVG using Scour. This removes some metadata and also shortens IDs as well as strip out comments. For the PNG files we used OptiPNG on the maximum optimisation setting. This can be slow on larger files, but for favicons should not take long. Here’s the before and after comparison of files sizes for a particular favicon, using the script:

I also recommend Inkscape, it also has scour built-in to it. When saving a SVG select the option "optimized SVG" which will give you options to pass to scour to lower the amount of markup there is. You can then do some hand editing after this to further remove any markup you don't want.

This project uses HTMLMinifier, optipng, and zopfli to create a custom production Elm build pipeline. You can see how I make use of these tools in this build script. Here are the results in case you're interested. I used the same ideas from this project to build and deploy dwayne/elm-conduit, which you can learn more about in my article Yet Another Tour of an Open-Source Elm SPA.

Did you compare to PNG crush and optipng? Those are what I used to use because they're easily installable Linux/brew packages.

It seems that the package was updated around 2019 May to not do that -- https://github.com/imagemin/optipng-bin/commit/6bb7f1c2edab67297c7d72fa0bd604ecc214b9f1 -- published as version 6, so maybe you're using an older one.

The script optimised the SVG using Scour. This removes some metadata and also shortens IDs as well as strip out comments. For the PNG files we used OptiPNG on the maximum optimisation setting. This can be slow on larger files, but for favicons should not take long. Here’s the before and after comparison of files sizes for a particular favicon, using the script: