it might be because of any reason you mentioned, I cannot really tell because I haven't used drupal before

but as for the memory, 10MBytes per load isn't too much I guess I've seen some other well known PHP scripts "vBulletin" using up to 64Mbytes on its own (I guess it depends on what stuffs are installed into it).

You shouldn't be having problems unless your system has something like 128MB of memory and those drupal process that consumes 10M are never shutdown (stuck process).

You can monitor the memory consumption by process using the "top" command ... pressing "SHIFT + m" will list those process by memory usage ... keep monitoring it to see if it's really about 10M per page or 100M as this zero would really make a difference.