While choosing the same font family, the font size is not the same across all text fields within (eg. legend vs axis vs axis title) and across visuals (eg card vs area chart),Same font family and same size should look exactly the same on the screen.
Based on my test with Power BI desktop August version, both area chart legend, X axis title and Card category label displays in the same font if we set the same font size and font family for them.
I would suggest you update your Power BI desktop to the August version then test again.
Best Regards,Qiuyun Yu