IDK why, but countries are making them bigger each generation
Not my area, but… I’m pretty sure there are rules and regulations on quantities of ships of certain classes in various waters - ie, one country couldn’t suddenly float 500 battleships in the Mediterranean. Frigates are the second smallest ship, so if you can shove tons of strength and firepower into the class of ship with the loosest regulation, then you can essentially get more firepower into the same spaces.
Countries have been fucking with the class designations (which, from my very casual understanding, are totally silly) to circumvent restrictions for many years.