It's smaller than a
Constitution-class yet seems to be built for the same general-purpose mission profile, which fits the 'light cruiser' terminology.
As far as 'pasting' a Connie goes, which version of the
Constitution-class weaponry are you using? The old fan-theorized 'only forward guns' version or the more canon version that does, in fact, have full weapon arc coverage? The TOS version or the TMP version?
There's also the never-talked-about-on-Star Trek-but-assumed-in-real-life factor that the
Constitutions, being larger, have larger guns.
RL light cruisers often had more guns than their heavy cruiser counterparts (which, ironically, often led to light and heavy cruisers having similar displacement) but they were usually 5 or 6 inch compared to the 8 inch guns on the heavies.
This could be a serious difference...at the Battle of the River Plate, the
Ajax and the
Achilles scored multiple ineffectual hits on the
Graf Spee, but the
Exeter, with her eight-inch guns, was fully capable of penetrating the German ship's armor.
There's also shield power, hull strength, etc to consider.
Of course, when analyzing the Star Trek II battles, keep in mind the
Reliant was an active-duty vessel whereas the
Enterprise had been relegated to training duty for some time.