Actually zetigrek it wouldn't suprise me much if Polish students move up in those test.
No actually class size has very little bearing on good performance, teacher quality is infinitely more important.
Because - I think the tests are measuring pure knowledge. They're not measuring things like problem solving - so Poland ends up artifically high because the education system here is very good at teaching "facts".
Actually it is the other way around. The tests are mostly concerned with problem solving skills. In case you ask, I know all this because I have been recently doing some education research for my boss-and methodology research not that fun):