Why does memory appear near full even after upgrade?

I've just upgraded my MacBook Pro from 4GB to 8GB. Before the upgrade, I looked in the Activity Monitor and it said that it was using almost 3.98GB of 4GB (I'm a web developer, so I usually have lots of apps open.) Then after the upgrade Activity Monitor shows that it is using about 7.50GB of 8GB (with the same apps opened.) The upgrade improved the performance of the Mac, it is surprisingly faster than before, but I was hoping to see more memory available after this upgrade. Thanks for your help!