In most of East Asia, where they test at higher math achievement levels than ours, class sizes are around 50. I don't think bigger class sizes will turn students into underachievers. Heck - undergrad college class sizes can be as large as 200 to a lecture hall.
It is always more complicated than it seems.
The best class i ever taught was 53 pre-calculus kids that were desperate to get into calculus.
At my school there are many teachers who can’t manage 20 kids, with 34 it will be a cluster &^%^.