Am I being stupid here....
We could work on finding a gentler word for it if you like?
if I bet 0.25BTC and roll 50.50 that's 2x payout, yeah which is 0.5 so how come my balance only increased by 0.25 each time?
You're effectively betting on a coin flip. Half the time you win, half the time you lose (let's ignore the house edge for the purposes of this - in reality you win slightly less than half the time).
You bet 0.25 each time. When you lose, you're down 0.25. When you win, you're up 0.25. So in the long run it balances out.
If you made 0.5 profit each time you won, and only made a 0.25 loss each time you lost, the site would soon go bust.
In short, the '2x' payout includes your original stake. You bet 0.25 and get 2 times that back when you win, so you get 0.50 back, which is your 0.25 bet plus your 0.25 profit. When you win, you double (2x) your money from 0.25 to 0.50 - a profit of 1x your money.
Get it?
. 
It increases by bet amount, not bet return. 