I think you will find that they didnt invest too heavily on one agency but spread it out over many.
I use Betfair daily laying horses only and the liquidity on harness at smaller meetings isnt big enough to plunge on in large amounts because you see it all the time 5k comes in to be backed at 2.6 then some imbicile puts in $50 at 2.5 and by the time it gets back to 5k @2.6 its been jumping from 2.4-2.5 for 10 minutes