Mac OS X, Memory Usage, and Web Browsers

Today while multi-tasking on my MacBook Pro, I ran into a few snags. I was getting ready to import some RAW files when all of a sudden my MacBook had slowed to a halt. I was watching a YouTube video in Safari, while syncing my iPod, and launching Lightroom 4. Behind those apps Twitter, Carousel, Wunderlist, and Evernote were running. I have 4GBs of memory installed inside on my MacBook, out of the possible 8GB.

Anyway, the moment Lightroom 4 began to load everything pretty much stopped responding: I was out of memory. I had to force quit Lightroom in order to regain control again. I checked my memory usage in activity monitor: 100mb of memory was free. What? This shocked me. Some basic apps were using up around 3.9GBs of memory.

While a handful of apps were running at around 100mbs, Safari turned out to be the real culprit. One tab, one video: 700mbs of memory was being used. For a web browser with a single tab open I found this to be pretty absurd. For the sake of comparison I opened up Chrome, resumed the same video, and opened a total of three tabs. Memory usage sat at around 600mb across Chrome's multiple processes. It seemed to be a bit more efficient.

After a quick "purge" command in the terminal, I managed to free another 1GB of memory. I found it odd that instead of freeing up the inactive memory, my MacBook had simply slowed to a crawl, and eventually stopped completely.

tl;dr: Is this normal behaviour? I am a former Windows user and I'm not used to a lot of typical system behaviour in Mac OS X, but this sort of inefficiency seems troubling to me. It often leaves me wondering what could be wrong?