Why don't you increase your trade size? You made 14% on £100. At that compounding rate you should be rich in no time. It sounds like you have the sports betting down to a bit of a science. That's why ask why not increase your trade sizes?
Because this is Betfair. The liquidity is limited. It dosent have the hundreds of millions that the index's or FX pairs have.. On an average day a race will have about £500,000 traded on it. So if you up your stake to much, you can end up not getting you bets matched and then you end up with some exposure in the race which amounts to pure gambling.
If i back a horse for £5000, then I want to lay it off at a lower price for £5000, yet I've only got 3 minutes till the race starts... I could find myself in a bit of trouble if my lay bet is still in the market and and has not been matched. Betfair cancel all unmatched bets when an event goes "in-play" ANd then the prices movements become wild.
If I straddle the market with £5000 each side of the current price, I effectively become the market maker. But i have no control over the balance of the market. I could get hit on one side and leave the other side totally exposed.
Bigger events such as the Cheltenham Festival, and the Grand national where you can up your stakes as there a a few million matched race. But as you can imagine, A Monday afternoon at Plumpton racetrack dosent quite generate the same level of interest.
As i've mentioned before, I've not quit financial trading, just spread betting, I want to learn to trade using price, volume and depth of market, and trade off the ladders direct on the exchanges. Betfair offers a perfect learning experience for this, without having to risk too much of your own capital.