Why? Well, let's look at the difference in performance..
When I bought the site earlier this year, it was on vB3, with a bunch of mods. In feb or march, I upgraded to vB4. That's not the surprise, though.
When I transferred the site to xenForo, the server is barely even hit. It's not even pushing limits. Even though that the site grew enormously. By the end of the year, the traffic exploded, yet the server isn't faltering with xenforo.
Pageviews, passthrough, the amount of kbytes read from server are all at rock bottom. While the monthly visits are skyrocketing. Now let's compare this with wordpress, which is where MW3Blog comes in:
You see the difference? MW3Blog is using an old skin, and yet it's pushing the server's passthrough numbers.
MVC3Forum is telling a similar story, and it's on xenForo:
I think what contributed to the high memory usage for MVC3Forum is that I have 2 images that take up the majority of the space.