Yes, classes under 15 weren't subject to the algorithm
Nothing to do with class-size, but cohort size.
If Eton enters 100 maths students each in classes of 4 people, they are still subject to standardization.
Also the threshold was not even 15, but around 4.
There was a secondary threshold of 15 to be partly used.
And it was based on the harmonic mean of the current year and past three years.
So for example, if you had:
2017 - 10 students in school for A Level maths
2018 - 10 students in school for A Level maths
2019 - 10 students in school for A Level maths
2020 - 10 students in school for A Level maths
then it's the harmonic mean of 10 and 30, which is 15, and in that case the weighting of the algorithm is (15-5)/(15-5) = 100%.
As an example, if you have 4 students each year forever, then that's the harmonic mean of 4 and 12 (they use 3 years), which is 6. In this case the weighting is (6-5)/(15-5) = 10%
Thus:
3 per year = CAG used in full
4 per year = 90% weight to CAG, 10% to algorithm
5 per year = 75% weight to CAG
6 per year = 60%
7 per year = 45%
8 per year = 30%
9 per year = 15%
Obviously with a brand-new subject you got full weight.
At Rye St Antony they appear to have had 21 students total in 2018. I guess a number of them will have had low cohort sizes. Anyway, all schools now get the CAGs for everything, so there will be bigger uprates for more students elsewhere, as Ofqual originally said 80% increase in A*s if they accepted the CAGs.