WordPress massive memory usage

I am using WordPress to run my website but I’m having serious issues with memory. I was on a shared host and it was using more than the 300Mb I was allocated. I have since moved to VPS with 1Gb of RAM, but WordPress is still using it all. That’s a huge amount of RAM being consumed. How on earth is it managing to eat up so much resources? It’s not even a particularly busy site and I have W3 total cache etc.. set-up.

What can I do?