Monitored with task manager:
Loading a 2.25 MB bitmap (2D image) leads to a memory usage increase of 50 MB.
Each subsequent -> "Close project" -> Reload leads to a memory increase of 10-25 MB.
After 3 reloads I got memory usage more then doubled, from 90 MB to 190 MB, with still only one node in the data manager.
Loading a 10 MB bitmap leads to a memory usage increase of approx. 120 MB.
A 29 MB 3D image gives 140 MB increase.

Seen on:
ExtApp 2012.02.99-4810dde9
Windows 7 64bit
Memory usage as given by Windows Task Manager
Reproducible with all tested 2D and 3D images.