Don't know how significant the figures are around the 15 races mark, as you can see below the number of 15 horse races is a fair bit less than the average for all fields.

Strike rates:
Qld 24.1% on 15 runners against 28.8% overall. (430 races out of 20,000)
WA 24.8% against 27.4% (455 races out of 8500)
Nsw 22.1% against 29.1%, (440 races out of 32,000)

I honestly don't think the 15 races being best is significant, more chance. I just ran Victoria and it returns 95.7% on 15 runners, strike rate 19.10% against 26.4% (730 races out of 25000).

Yes generally the selections are in the top three of favoritism, I have had a $20 winner or two but they are red letter days indeed.
By the way, if I bet winners I bet to a price and find that this cuts the bets by about 50%, cuts the strike rate by 5 to 8% and boosts the ROI by 7.5 to 11% (depending on the state). Doesn't give any more meaningful figures by field size though.

KV