Mr. Logic is absolutely correct. The bigger the dividends, the more the TABs keep for themselves. Presumably they assume punters either don't know or don't care because of the size of the dividends. So more money is taken out of first four pools than trifecta pools. Significantly more money is taken out of trifecta pools than win pools.

Using a favourite as a roving banker for trifectas in small fields just about guarantees you must lose a significant amount of money long term.

As a very rough guide the best trifecta races to bet in are:
1. Fields of 11 to 14 runners
2. The favourite should be at least $3.00, preferably longer odds.