Yeah the trees are fat, but they aren't particularly tall. Toady did talk about 30z or 20z tall trees being possible, but that was before he implemented them. 27z would be like the Grant Tree.
Which has a trunk diameter of about 5m, right? What're the dimensions of a tile meant to be again?
With pulping coming to prevent Necromancers and reanimation to be too strong, do you plan to ever add a limit to the number of corpses a necromancer is able to reanimate at a time ? With something like a necromancer skill to increase our corpse cap and/or our range. And making reanimating creature that are far stronger than us possibly go berserk because of its strong will being able to be partially free from our control over them, but not being strong enough to be sane.
From the wiki: "Necromancers who have a sufficient following may use their zombie slaves to build dark towers, a task that requires at least 50 followers"
http://dwarffortresswiki.org/index.php/DF2012:NecromancerDF doesn't seem like the type of game that wants to allow the AI to do things that the player couldn't theoretically do so I assume you would be able to get at least that many followers and probably more. Of course there's a difference between directly controlling that many zombies as a fighting force and simply animating them like puppets. So there's merit in considering limits on how it would work and a sense of the "strength" of different necromancers that is not tied exclusively to how many zombies they curently have. But that's when questions start becoming suggestions. I'd guess that Toady hasn't thought about it much yet.