Have never win big to really donate some, but why not. If I win a lottery one day, I would first like to get my life on track, give some to my parents , help my brother with his studies and then if i am left with any, I wont mind donating it for a charitable cause. Good deeds never hurts and also gets back your good karma !
Only if someone wins big can actually think about donating it or else generally people calculate that out of the past losses which they incur and the amount if they have won now still how much losses they need to recover or have they become a positive balance sheet and decide accordingly.