I had to take my macbook in for a hard drive replacement a few months back.
When I got it back it felt much heavier.
However, I checked it against other people's macbooks and there was no change.
I think its all in your head.
In answer to your question, google tells me that the original macbook weighed 5.14 pounds and the apple site has the new macbook at 5.1 pounds. So no, there's been no change.