Huh.
I open a project. I insert 45mb object, I create a 10k population. Memory at 1.5gb. I change to wireframe, memory at 1.5gb. I change to textured, memory at 1.5gb... this object is also having it's twigs turned grayscale and PF colours added so should be extraneous on memory I'd think.
Terragen does have a pretty substantial memory leak, perhaps it's related there and something specific with the objects.