as dense as architecture can get?
Rick asks a number of questions regarding Speer's plan, plus he raises the issue of proper comparative scale. First, I'll leave the issue of proper scale aside until I find my photocopies of Speer's [Gross-Berlin] plan (and if anyone has access to Leon Krier's book on Speer, that's where I got the copy of the plan)--it's there that the comparative scale of Speer's north-south axis can be seen in context relative to Unter den Linden.
Unter den Linden crosses the Speer axis directly in front of the Big Dome's forecourt, that is, just south of the dome's forecourt. The bend in the Spree River then curves around the back of the large dome. Now that I think of it, Spree did not eliminate the Reichstag, but in fact incorporated the (still existing today) Reichstag as the east side of the big dome's forecourt (I'm pretty sure this is what that plan from Krier's book indicates, and you can see a slice of the front of the Reichstag's footprint creating the (upper side of the) forecourt in the image Rick sent). And comparing both Speer's axis and the Unter den Linden plan within the plan comparisons Rick sent, it looks like they are the same scale--in the Unter den Linden plan you can see the southern half of the Reichstag's footprint, which is just northwest of Parisier Platz/Brandenberg Gate.
As to wondering about the 'easy' play with scales relative to Piranesi's Campo Marzio, in part, Rick, you guess correctly. I say in part because when Piranesi delineates the Campus Martius proper, he more often than not uses the correct scale for the buildings that once existed there. Piranesi grossly exaggerates building scale in the Campo Marzio's outer regions, however. Nonetheless, Piranesi is deliberately 'playing' a learning game here, in that the outer regions is where Piranesi's plans and programs lack practially all veracity, hence, the hyperbole of Piranesi's architectural imagination is coded by a hyperbole of architectural scale. In simple terms, the over-sized plans of the Campo Marzio indicate buildings that Piranesi completely 'made-up', where as a high percentage of the smaller building plans indicate buildings that actually once existed and are drawn in their proper scale. (Mind you, the drawn plans of the once-existed buildings, even though at a correct scale, are still most often individual plans of Piranesi's invention.)
In case it isn't already obvious, I have something of a fetish for reading the plans of places that don't exist. Kind of an inversion of deja-vu all over again. Or is it a reenactment of deja-vu all over again?