In case you've been living under a rock for the past two years: Drupal 7 is known to use quite a bit of PHP memory everytime a page is loaded. I won't get into anything why that is and why it's such a big jump from Drupal 6 (I'm in no position to comment on that), but what you need to know is that Drupal 7 can easily use 40-50 MB per page load for a small to mid size website, and much much more for larger websites. Turn on some sort of PHP opcode cache like APC and you can probably get that small to mid site down to 10MB of usage or lower.