It's likely to be due to the fact that a) the season has just started so all accounts drop down to a lower league - hence lower DEC per win ; b) that you only have a rating of 100 (don't worry, this will increase as you win more battles)

When I first started I was only winning 0.01 DEC per battle - but it does grow. Keep at it. You will see it start to get to 0.5 and 1 DEC soon enough... and then exponentially get higher.

I'm getting around 15-20 DEC per win in silver league. The big boys in gold, diamond, etc earn a lot more per battle (and also get more cards per "daily quest, etc). It really does snowball after a while.