Of course this changes, because the work per student head increases. The admin staff have to process more work. The teachers have to do more marking. The school spends more money on many of its resources. It needs more classroom space, more chairs, more loo paper, more practical time, more teaching assistants, more time for pastoral matters, more supply teachers, more stationery (yes many schools do still provide this, and textbooks).
Your post seems to just assume you can increase teachers’ workload because they have “spare capacity” 😆 Ask a teacher - well, anyone really - do they want to do more work for the same pay? More marking, prep, etc. for the same timetabled lessons? And ask them if adding a bunch more kids per class will have no effect on the quality of their work? What answer do you expect to get? Do pupils and their parents favour schools who offer larger class sizes at A-level or smaller ones? Why? Are there lots of kids clamouring to be taught in A-level classes of 30? Why not?
@noblegiraffe you still haven’t said how many extra pupils per class you’d be happy to take in with no timetabling offset for marking time. Are you happy to mark five or ten more students’ work in the same time you already allocate for existing marking? Will adding more students to a class increase or decrease the quality of your teaching? Will you be giving each student the same amount of individual time? (A-level maths is hard, and many students struggle. Do they just get less time and attention each if there’s more in the class? Is that what you and they want?)
Would your colleagues in English, Psychology or Biology be keen to add more marking to their workload with no offset or extra staff?