Hi Sportz,
With 8 runners exactly, all sorts of wild things tend to happen. For whatever reason.
I recon one could bet every double digit runner in the race & still be no worse off betting the best form horse in the field , provided there are no odds-on runners in the field.

With 9 runners exactly , one is competing with 8 others.
Must be $3.20+ SP or PP.
The form horses seem to be able to do their thing more.

With 10 runners exactly one is competing with 9 others, a 11% difference.

As a test, run any reasonable selection process over races with exacly 9 runners .

Then do the same with other size fields of 10 & see if there is much difference.

I think one will find the SR slightly higher for 9 runners.

Just for fun, I did this...
I ran over the results for Sat28th Sun29th Mon30th July.
. Races with exactly 8 runners.

. No bet in races where the fav is $2.00 or less.

. Sometimes there were up to 5 horses bet at a time in the same race.

. Betting all runners $10.00+

4 wins from 13 races = 31% SR
O/L 46 ret 56 = Profit +10 = 22% POT
No form - No nothing.

Prices $17.50 10.70 10.70 17.80
Most of the form horses with nostrils flaired & flatulating just before jump , had their butts kicked.

Cheers.

__________________
Cheers.