From what I've seen at least the files are much more organized (there are dedicated includes and such). Obviously I can't tell about templates.
Supposedly vB3's database is half the size of a vB2 database, but the files are twice the size.
I can't say the files are twice the size, they are more wide spread to reduce load and size during load, hence the amount of files now in 3.0 so far.
The queries have also, from what I have seen, have been almost cut in half. A regular forum home is boating 10-13 queries I think, down from its normal 18-20 (last I checked).