Well multi-tile trees should be easier than multi-tile creatures, because trees don't have to do much pathfinding.
The difference is much more, in fact, I'll say that there are no direct relations at all
The giant trees I am talking about here are not actual entities (as in identifiable by the DF program at the moment as a tree). In fact, for a fast implementation, all it takes is just some new "woody" material (stone type, not log) and the world/map gen to generate the trees as a part of the terrain. (Disregarding tree dynamics like root sucking water, leaves sprouting etc). It is not a revolution as per say, but just a sort of smart reuse of existing features. E.g. imagine a magma pipe, turned all to solid woody material and moved halfway out of the ground, then have some sideway branches generated near the top. From there, they just become part of the terrain. No rocket science at all.
All I have to say, aside from the fact these trees would be pretty cool, is that sphr's recent comment was frighteningly long.
Comes from decreased frequency and batch processing. If you take the average length per person replied, it's not that bad. most of those are quotes anyway... (tries to look innocent)
These are already planned I am pretty sure, but it might be a long time before you see them. I remember in one of the many FOTF threads I once asked about this, and Toady himself said that he would like to eventually have giant z-level spanning trees with walkable branches, etc. and Elves building cities amongst them. IIRC.
Indeed, then I shall try not to say much more. But it COULD be a means of bumping priorities for such features. Esp when doing naive analysis, (details cannot be known since I don't know how Toady programmed his stuff), the basic stuff for implementing a working form of giant trees is already in place, disregarding some possibly new dynamics (e.g. allowing un"mined" walls of "woody" material to burst into flames and spread to nearby un"mined" tiles, which could be something not handled yet, or I could be wrong) The only thing that needs to be programmed is the map generation part which is atm, not user-moddable.