Part of the equation is wheelbase and weight balance between the wheels. Smaller frames often get shorter in front to reduce the reach. If the front wheel comes back, then if you want to keep a proper weight balance between the wheels, yo u need to bring the rear wheel forward, This means reducing the clearance between the tire and the seattube or FD clamp. Solution? Steepen the seattube or bend it. Not a lot of fans of curved seattubes and the bike's gotta sell.

And the seatpost/setback issues? Changing seatposts to a larger setback post is easy. (Well on any bike that doesn't use a proprietary seatpost and tube or mast.)

I have my customs built with steep seattubes (74 and 75) for weight balance and run custom 60mm setback posts with the clamps centered on the rails. Bikes handle great and the ti railed seats love being centered, (Slamming them means breakage at over $100 each time and a lousy ride home. I also make sure the clamp has a "soft" edge, especially at the lower back. Use a file on it if it doesn't)

Ben