I haven't been following this space in a while. Can you shed some light on why the sizes are changing from 14px to 16px? On the kickstarter page it says it says the new size matches Bootstrap 4's global font size; is that the sole reason? (Edit: this was partially answered since but I'm curious)
I wonder, is traditional incremental versioning appropriate for what users perceive as a 100% stylistic change (notwithstanding the immense effort of a full rewrite of its internals). Do you intend to enhance Font Awesome 4 after the release of 5, or is 4 going to be EOL?
It has been 16px as default for as long as I can remember. People designed at 12-14px and it has been trending larger, 18-22px, for quite some time now - at least since 2012-2013. I think larger screen sizes plays a big part in the change, or maybe marketing people realized people dislike small text/prefer larger text? :)
Coders have learned that it's best to not touch the default font size, or resize html/body font size. The ugly 62.5% (16-->10px) is also dying out for accessibility reasons.
Oh, and the icons aren't that consistent. It happens over time as you do 40-ish icons per release. It wanders a bit and you lose consistency.
We're fixing all that.