People need to stop treating these categories as some sort of fixed regime that lawyers are subject to. It is after all just a job much like any other job in any other industry at the end of the day, where employers are looking for individuals with certain skill sets. You develop your own set skills and bring them where employers seek.

You don’t magically get to transition from “mid sized” into “big4” or vice versa as a rite of passage / some sort of career progression normality (although admittedly headhunting is fairly prominent). It very much depends on the skill sets you possess, your career goals and what employers are looking for. Imo you’re doing yourself a disservice by treating your career as some sort of tick boxing exercise.