The less returns in the big fields interests me. I can offer you one POSSIBLE explanation: small fields tend to almost always be slowly run. They dawdle. In slowly run races, horses who shouldn't win can because of getting the run of the race, the right ride and falling in. In big fields, truly run affairs, the best horse tends to have it over the bad one. No doubt you've heard the race callers making that great racing generalisation, in their peculiar nasal tones - "Well, how often do you see it. Small field, and the outsider gets up." Slowly run races are a farce, and they mean form doesn't become the most important thing, where they are positioned does.
To illustrate the point: 16 horses running over 1600m-2000m at Flemington, in a truly run affair, the cream rises to the top. 7 horses in a crappy class6 at MV on a Thursday night and anything can win. I have seen 1000m races at MV on Thurs nights which have been crawled in front.
A good example of a jockey counteracting this was Jason Benbow on Highclere on Saturday. Whilst he rolled along at a good pace, a true pace, the kind of pace conducive to running the best times, the rest of the field sat 10-12 lengths behind him. THEY were in a slowly run race all of their own while he was running a race the pace it should be. Well, evenly run races always run better times than slowly run ones, so little wonder the other jockeys found themselves scratching their heads after the race, wondering how he had managed to keep going on Highclere. The horse wasn't going too fast, just so ingrained in the minds of these uneducated dwarfs is the culture of the slowly run race that they no little else, and have no imagination to deal with it.
You know what they say - if jockeys were one foot taller, they'd all be unemployed.

Hmm. Just re-read all that. Kind of got off the topic. Sorry..
Duritz