Dale,

Very insightful post. Lets back that up with some more tests. I'll use tote figures only.

Rule 1 : Top unitab selection

Field size < 8 had 910 selections for a return of 729.6 = 20% lot
Field size 8-12 had 5911 for a return of 4975 = 16% lot
Fieldsize > 12 had 7627 for a return of 6510 = 14% lot.

As you predicted removing smaller fields produce a less loss or greater profit.

The reason I believe this the case is that the extra horses are generally no hopers anyway, but by having extra horses their is extra risk thus increasing the horses odds by more then the actual added risk.