Not strange since internal memory is completely different from random access memory, or RAM.
If you use a third party application such as Tasky (free) or Handy TaskMan (not free), then you can accurately evaluate how much available RAM your phone currently has and which applications are using it. I'm not familiar with the E50, but this isn't new to 3rd Edition. The web browser is perhaps the most RAM-intensive application ever put on a s60 Nokia.
Use either of the above applications to determine what applications may still be running if you failed to close them. On the N73 in particular, some default applications are left open intentionally WITHOUT appearing in the default task manager (holding the menu button) so that they open faster when accessed.
As far as text messages go, the only reason this would have an impact on RAM is if the application itself became dramatically larger to run due to an increased size. Keep in mind, we're talking about two different types of file size that influence two types of memory. The disc size of the application is affected by the amount of messages in it, thus the RAM is affected because it will take more RAM to open a larger application.
Does this make sense?
The best you can do is keep on top of which applications are running, keep them closed, and reboot at least everyday. Rebooting clears all RAM and brings it back up to where it should be.