I doubt the key is lighter, did you weight it? Also, what switch do you have?
I'm assuming the pressure required feel less because of the fact it's the largest key on the entire keyboard. I don't understand what you mean by 'it feels like a kids toy', that's a bit subjective.
In the end if the spacebar feels so wrong to you, there keycaps for sale. You could get a heavy one made of ABS?
I personally enjoy typing on this keyboard, I dread going back to work on the horrible rubber domed KB we have. Probably going to end up buying a K70 for work.