I am using the default graphics device. I've tried changing dpi and fig.retina. Neither have any effect.
However, decreasing fig.width and fig.height seems to help. I had been using fig.width=12, fig.height=8 (for years), e.g.
knitr::opts_chunk$set(comment="",results = "hide",echo=FALSE, fig.width=12, fig.height=8,
include=TRUE);
Switching to
knitr::opts_chunk$set(comment="",results = "hide",echo=FALSE,fig.width=9, fig.height=6,
include=TRUE);
has restored graphs to something like what I had prior to upgrading.
Something related to scaling must have changed but I can't figure out what. At least my graphs are legible again, but I'd still like to know what changed.