The majority of the WW1 tanks were done off line drawings by the same person, such as the FT-17, British Tanks, etc. The line drawings were all the same scale and thus reduced at the same scale.
There are at least three scales listed with 15mm, and I used the larger of these scales to make the tanks.
Now in terms of size, yea, they are most likely larger than some of the other companies. As to being wrong when done in scale, that I will debate. I had one person say I did the Whippet wrong because it was taller than the German tank. If you read the dimensions of the two tanks, it is taller than the German tank by 2 feet (I believe, as I don't have my books near me).
I have talked to a gamer in the Mpls area that uses Peter Pig to do SCW and he uses the Blue Moon tanks. His response to how they fit, "Perfectly", his words not mine.
And yes, our buildings are on the larger size because they were made for figure lines that work for skirmish games. Thus, I didn't want doors that were shorter than the figures. Plus, I wanted people to be able to move the figures about inside the buildings. Why create lift off roofs if you can't get your hands inside the buildings.