Re transitioning between small and large businesses (or tiny, small, medium, large, massive - as desired) in a fluid manner, how about kicking in the greater responsibilities according to ranked pay.
<=50 employees (say, for the sake of argument, and this is inclusive of management) require certain responsibilities supportive of the small business and encouraging employment, and does not demand employer-contributions or anything more than the statutary minimum employee PAYE (or whatever you lot do, over there). For 51+ employees, there's additional business taxes and things, but (instead of kicking straight in) it's applied as a percentage-point of the intended additional income-related taxes upon the 51st (and 52nd, 53rd, etc) per >50 employee, until hitting 150, when the 50 most menial labourers (cannot be Board Members/owners, to avoid with tax-avoidance, but with the caveat to allow for Workforce Representative appointments where they exist) are still benefitting from minimal tax burden, the other 100 are paid well enough (and clearly earning the company enough) not to significantly notice the additional burden asked to bear, or at least finding a comfortable level where gaining an employee is preferable to shunning additional business that might only be serviced by that employee being there to cover the ground.
Then, for 150+, a further bracket kicks in (0.1% of bracket-requirements, per employee, until 1150?) and again (0.01% per employee until 11.15k employees).
Not perfect. Needs some way to deal with undue splitting of large firms into unwarranted 'franchise' components of what should be a whole, and other things, I know.
Also, I started this post with the idea in mind that the term "geometric mean", as applied to take-home-wages, would apply somehow (or something similar, so that "geometric upper-quartile" would apply if a theoeritcal 25% of the workforce should be asked to be the higher contributors, but I momentarily forget what the "nth-percentile, geometrically adjusted" might be called) to help flatten the high/low disparity. And it needs a better economic-mind than mine to help avert even the more obvious ways of tricksing the system.
Plus things like "below n employees, you need only produce self-certified accounts upon request; above that, you need certification by a trained accountant; above m employees, further oversight is required by State/Federal regulators" are tricky to phase in, especially when trying to avoid short-changing by borderline employees using zero-hours contracts or 'contractor' status to hide true-employees from the count. But there'll be answers to, this, already. Probably the ones most argued against in legislature (by those lobbied upon by the people who would miss their favourite loopholes). Perhaps we should study failed amemdments to employment law more even than the passed ones?