i agree with you if you invest your money in big amount in trading then surely you will be get a big profit from trading and if you invest your money in low amount then you will be get a low price .
It is normal because profit will be proportional to amount invested or being used for trading.
You do not expected Mr A to invest 10BTC and Mr B 5BTC and they all make the same profit, that is verry unusual because profit is always going to be proportional to amount invested and also in other wards risked.