Firefox's telemetry has data on how many times a CA is used to successfully validate certificates. This is a pretty good measure for how "big" a CA is. The data is hard to view in Mozilla's site, so I've made a script to combine a few data sources and graph it! https://github.com/mcpherrinm/cert-count

Of all the things I didn’t expect to ever happen, iOS Safari actually got a certificate viewer in 18.4! https://webkit.org/blog/16574/webkit-features-in-safari-18-4/#connection-security