On my system, with Iceweasel (Firefox) 24.3.0, the effective body font size decreased. This appears to be due to the change in effective body font from DejaVu Sans to Nimbus Sans L. It seems the font family change means it's actually smaller despite the slight em bump.
I'm not sure what a feasible solution to this is, but I'm putting it here for tracking.
I'll attach screenshots from English Wikipedia, but the issue is also present in core (I compared wmf/1.23wmf19 to master (768c1872435a94ec936c982dd72cb0074b7ea6fc to verify it was present in core itself).
Version: unspecified
Severity: normal