Why are small fonts the norm in research papers? Is it just me or are the published articles a strain on the eyes in general? Has there been(/shouldn't there be) a study on optimal font sizes for reading?
This is a historical anachronism. The fonts are chosen for the print version of the journal.  For a print version, if you use a smaller font, you can squeeze more text onto the same page and print the same paper on fewer pages -- which saves money.  So, for print publications, the small font size arguably makes sense.  However, today print is less important and the digital format is more important, but journal requirements haven't caught up to this fact.

For instance, in computer science, the ACM is a notorious offender: they require papers to use 9pt fonts, which are very small.