You had me scratching my head for a minute there, ROO, we're going to need an acronym glossary soon.
I don't isolate races by field size when I bet so it never really occurs to me what the run of outs is on any particular field size. Maybe I'm having a longer ROO on those races but hopefully a 5 or 6 horse race will come up in the mean time and ease the pain.
If you run the strike rates through a ROO predictor (23%ish against 29%ish) that would give the likely run of outs on those races compared to the overall figure.
Funny, I just ran NSW figures again and I get a strike rate of 35.6% in 6 horse races (2,135 of them) and 25.4% in 12 horse races (3500 of them). Now there's one for the statistics guys. Why don't you get a strike rate twice as good with half as many horses. And how can we use this to our advantage? 14 Horse races 23.7% against 7 horse races 32.9% - same question.

KV